Role of starch in the ceramic powder synthesis: a review

Sol–gel process is a method of the production of inter alia ceramic powders. It consists in the slow dehydration of the hydroxide sol of a given powder, resulting in a gel, which then undergoes the calcination process and turns into fine and uniform powder. The advantage of this method is the relatively low production cost and the possibility of obtaining various types of materials. On the other hand, the disadvantages include the difficulties associated with the controlling of the particle size and the degree of its agglomeration. This problem can be minimized by applying either various capping agents that block the growth of grains or polymerizing agents that support gelling process of the system. Currently, the most commonly used for this purpose are polymers of natural origin, mainly starch and its derivatives (Fig. 2) [19‐30].

Zak et al. [19] used the sol–gel method with the presence of starch as the stabilizer to control the cation mobility and growth of the resulting ZnO nanoparticles. According to the increasing calcination temperatures (400–600 °C), ZnO nanopowders of 28–51 nm and of hexagonal structure were obtained. Balcha et al. [20] obtained similar results. Using a calcination temperature of 600 °C, they got crystallites with a size of 28 nm and of hexagonal wurtzite structure. They also proved that the sol–gel method is better than precipitation, because the obtained crystallites are smaller and their photocatalytic properties are better. In turn, de Almeida et al. [21] received ZnO crystallites of 20 nm, using the calcination temperature of 500 °C by 2 h. They also concluded that the cassava starch used as the stabilizing agent inhibits the crystal growth, thanks to the interaction of the C–O groups of the glucose (resulted from the starch hydrolysis) and zinc cations.

Darroudi et al. [22] produced crystalline nanoceria powders by the sol–gel method in an aqueous solution. Long-chained starch terminated the growth of nanoceria particles during the reaction and stabilized them by covering the powder nanoparticles with hydroxyl groups as a capping agent. The resulting cerium hydroxide samples were further calcined in the temperature range 200–600 °C for 2 h to obtain CeO₂ nanopowders of about 6 nm with a cubic fluoride structure. Ferreira et al. [23] obtained nanoceria powders of ~8–16 nm and of fluoride structure using cassava starch as the chelating agent and calcination temperature 200–500 °C for 1 h.

Alishah et al. [24] investigated the antimicrobial properties of the copper oxide nanopowders produced via the starch-assisted sol–gel method. The temperature of the gel formation in this process was 65 °C, whereas calcination temperature was 400 °C. The utilized carbohydrate helped to received stable and spherical-shaped nanopowders, whose average size was 54 nm.

Khodadadin [25] compared the structural, optical, and photocatalytic properties of the undoped and Nd–Ce-codoped titanium dioxide, where starch was used as the “green” modifier that prevents agglomeration, limits excessive grain growth, and in conclusion modifies the photocatalytic activity of the synthesized nanopowders. He also revealed that small amounts of dopants (up to 1 mol%) improve the photocatalytic activity because they reduce the bandgap. Higher dopant amount (2 mol%) means occupation of the active surface sites by the Ce and Nd particles, and as a result, disturbs the photocatalytic effect.

Zhang et al. [26] compared the capping/stabilizing properties of acrylamide and starch/glutaric dialdehyde in sol–gel synthesis of alumina nanopowders. Both of the mentioned systems helped to form a matrix (without any inititators and cross-linking agents, just self-polymerization in temperature of about 100 °C) for the entrapped metal ions and gave rise to alumina particles during heating. In both cases, the formed gel was dried for 36 h at a temperature of about 95 °C and then calcined at various temperatures from 350 to 1200 °C. The difference was in the heating temperatures in which alumina phases started to occur. For acrylamide, a g-Al₂O₃ phase began to appear already at a temperature of 650 °C, whereas for the starch/glutaric dialdehyde it was 804 °C. The phase system from g to a started at 1100 °C for both systems, and a pure a-Al₂O₃ phase appeared at 1200 °C. What speaks more for the starch/glutaric aldehyde system is that in this configuration, this monomer is less toxic than acrylamide itself.

Koferstein et al. [27] synthesized BaCeO₃ nanopowder in the sol–gel reaction with the use of starch as a polymerizing agent. The obtained (BaCe) gel, after heating up to 920 °C, transformed to pure BaCeO₃, which was characterized by a specific surface area of 15.4 m²/g and a crystallite size of about 41 nm. This method was also used to obtain LaFeO₃ nanopowders exhibiting improved magnetic and optical properties [28, 29]. Calcination of (LaFe) gel at 570 °C allowed to obtain the pure phase of LaFeO₃ powder with a specific surface area of 25.7 m²/g and a crystal size of 37 nm, while increasing the temperature to 1000 °C, Tizro and Kamali [30] used the sol–gel method to obtain PbTiO₃ nanoparticles subsidized with silver. In addition to starch, glucose and lactose were used as the gelling agents. Analyzing scanning electron microscope (SEM) images, it was found that in the case of glucose, PbTiO₃ powder was compacted into micrometric-size agglomerates. Using lactose, the number of agglomerates was reduced, and nanoparticles in the range of 200–400 nm were obtained, whereas in the case of starch, spherical particles with grain sizes of 30–50 nm were obtained.

Starch, thanks to its structure, i.e., long polymer chains, single-helix structure, and a-glucose units, covers the surface of nanograins by hydroxyl groups, becoming the capping agent and matrix for the nascent powders, preventing their excessive growth. In addition, thanks to the presence of a large number of hydroxyl groups that have the same properties as those coming from alcohols, starch can be used as the oxidation agent for metal salts, i.e., it is a good polymerization agent (Table 1).

Combustion synthesis, i.e., self-propagating high-temperature synthesis, is one of the most efficient and economic way of obtaining ceramic powders, due to its simple experimental route, relatively fast procedure, and what is the most important, high-purity products. This method introduces high reaction temperature and specific heating/cooling rate, which helps to control the microstructure of the powders. In typical combustion synthesis, the first step involves the reaction between an oxidant precursor (usually metal nitrate) and a fuel (organic derivative) that is a reagent reducer. Energy produced as a result of the reaction sustains a further process, resulting in the ignition of reagents and the formation of a ceramic powder. The resultant products are metal oxide or its spinel, carbon dioxide, water vapor, and nitrogen. The crucial parameters here are the type of the combustion fuel and fuel-to-oxidation ratio. Selecting the fuel, we should remember about its enthalpy of combustion. The lower this parameter is, the lower the exothermicity of the combustion process, and the lower the temperature at which the selected powder is synthesized. In the past, hydrazide-type compounds have been used, but nowadays, because of its nontoxicity, mainly urea, carbon, and polysaccharides are used, among which the most popular is starch. For example, combustion enthalpy of urea (DH_f) is −147.26 kcal/mol, whereas that for starch is −401.57 kcal/mol. Considering the ratio of fuel to the oxidant, three cases are possible. Stoichiometric (described above), fuel lean, in which oxygen is in excess and it appears in exhaust gas, and fuel rich, in which oxygen is deficient, so a portion of atmospheric oxygen is necessary to complete the reaction [31‐33].

Tahmasebi and Paydar [32] studied the effect of starch addition on the solution combustion synthesis of Al₂O₃–ZrO₂ powder. Because of the large heat of formation of starch molecules, they replaced urea with starch in the mixture of fuels, and in this way, reduced the exothermicity of the combustion reaction. This prevented the sintering of particles and resulted in fine agglomerates of the Al₂O₃–ZrO₂ composite powder crystallites with a diameter less than 20 nm.

Qui et al. [34] examined the possibility of the production of AlN powder from combustion synthesis precursors, such as carbon black, glucose, sucrose, citric acid, and water-soluble starch. When the carbon source was carbon black, pure AlN powder was found in the sample calcined at 1500 °C, whereas for the precursors based on water-soluble organics, pure AlN powder was already found in the samples calcined at 1400 °C. Comparing the particle size and shape, it was found that AlN powders synthesized with the use of inter alia starch, had fine spherical shape and particle size of about 100 nm, whereas those based on carbon black were larger and their size distribution was wider.

Bai et al. [35] used the solution combustion synthesis to obtain MgO powders. The fuel-to-oxidation ratio was the crucial factor and because of that, various redox mixtures with starch-to-magnesium-nitrate mole ratio of 5.5:12, 5:12, 4.5:12, and 4:12 were examined. During the combustion process, nanoporous agglomerates were formed and the average crystalline size was about 8 nm. This value was growing up together with the starch content. They also used [33] the same method to obtain MgAl₂O₄ powders. As the fuel for the process, they were testing urea, glycine, and starch in various combinations and ratios. Their tests confirmed the positive effect of starch addition (0–35.6 wt%) on the specific surface area (from 13.36 to 35.53 m²/g) and crystallite size (from 200 nm to even 15 nm).

Raja et al. [36] compared the processing properties of starch, urea, and glycine in microwave combustion synthesis of Co₃O₄ nanopowders. Instead of traditional heating, they used microwave energy (2.45-GHz multimode cavity at 850 W for 10 min). The average crystallite sizes for the starch, urea, and glycine-assisted cobalt oxide nanopowders were 23.3, 26.9, and 30.2 nm, respectively. Another significant difference was in the shape formation of the obtained nanoparticles. Starch-assisted grains were nanorods, whereas urea- and glycine-assisted ones were spherical-like and nanospheres, respectively. In turn, magnetic measurements revealed that if the fuel is starch, cobalt oxide is ferromagnetic, whereas if the fuel is urea or glycine, cobalt oxide is superparamagnetic.

Visinescu et al. [37] used the low-temperature combustion-based synthesis to obtain zinc aluminate oxides. Starch was tested as a single fuel and in a two-fuel mixture with N-methylurea in various ratios. The combustion process started in the temperature range of 170–450 °C, but the pure spinelic phase was obtained at 800 °C. It was proved that the presence of starch helped to lower the temperature and increased the time of combustion reaction, i.e., starch decreased the process exothermicity. Starch-derived zinc aluminate oxides were in the form of porous aggregates in contrast to N-methylurea-based denser forms.

To obtain BiFeO₃, Koferstein [38] used a combustion method supported by starch that served as a complexing agent. (BiFe) gel was calcined at a temperature range of 500–750 °C, and pure-phase BiFeO₃ powder was obtained already at 550 °C. Further increase in temperature caused gradual grain degradation to Bi₂₅FeO₄₀ and Bi₂Fe₄O₉. This process was also dependent on the degree of heating and the fuel/oxidizer ratio.

Khattab et al. [39] used a modified combustion method to obtain Co_xMg_1−xAl₂O₄ blue pigments. The output powders were prepared as a result of the microwave combustion method. Then, to optimize the spinel formation, the resulting powder precursors were calcined at 400, 550, and 800 °C. Starch used in this process as a combustion fuel caused the powder precursors to become oxidants. In addition, by analyzing the DTA and TG curves, it was found that the temperature degradation of starch contributed to the reduction in particle size of the powder.

The above studies confirmed excellent starch properties as combustion fuel among different organic derivatives such as urea and glycine. The more carbon in the precursor and the lower combustion enthalpy, the better properties of the ceramic powders synthesized by the combustion process can be obtained (Table 2). Due to the fact that starch can provide a lot of energy to the system during combustion, as a result of which the exothermicity of the reaction is reduced, the temperature at which the powder can be synthesized is also lowered. Despite these important advantages, there are a few things to keep in mind that can disturb the starch-assisted combustion process. It should be remembered about the mass ratio of starch to metal nitrate, i.e., fuel-to-oxidation ratio. If it will be lower or higher than the stoichiometric one, we will get impure powder or a fully nonsynthesized one.

Besides combustion method and sol–gel process, ceramic powders can be obtained by the use of other methods, mainly combined ones. Starch here plays a role of the capping agent and/or carbon source. For example, thanks to the combination of the sol–gel process and combustion synthesis, fine ceramic powders, mainly spinels, with high homogeneity and purity, can be obtained. The biggest group of the powders that can be obtained via the starch-assisted sol–gel combustion method are ferrites.

The starch-assisted sol–gel combustion method was used by Koferstein et al. [40] to obtain MgFe₂O₄, which was then subjected to isothermal sintering in the temperature range of 550–1100 °C for 1 or 10 h. The resulting sintered materials exhibited varying magnetic properties, depending on the particle size, which could be adjusted using starch. Yadav et al. [41] synthesized NiFe₂O₄ nanoparticles by the use of this method, where starch acted as the source of organic fuel. Gel obtained from the sol–gel process was heated up to 200 °C to initiate a self-sustaining combustion reaction. Such obtained powders were annealed in air atmosphere at 200–1000 °C. X-ray powder diffraction (XRD) analysis showed the improvement of crystallinity with the increment of annealing temperature. Also, the higher was the temperature, the bigger size of particles was observed, starting from 5 to 15 nm at 200 °C (spherical shape), up to 70–300 nm at 1000 °C (spherical and elongated shape). It was also proved that better crystallinity can be achieved by a proper ratio of starch and nitrate. They have used the same method to synthesize Co_1−xZn_xFe₂O₄ spinel ferrite nanoparticles [42], Nd³⁺-doped CoFe₂O₄ spinel ferrite nanoparticles [43], and ZnFe₂O₄ spinel ferrite nanoparticles [44].

Dinh et al. [45] studied the effect of annealing temperatures (700, 800, and 900 °C) and Cu²⁺/Fe³/starch ratio on the magnetic and catalytic properties of CuFe₂O₄ powders. The higher the annealing temperature was, the more tetragonal spinel structure phase was observed and the less impurity phases like CuO were observed. Increasing the starch content in the examined systems, it was found that it does not affect the morphology of samples, but the particle size (from 100 to even 500 nm) and the photocatalytic activity. The optimal annealing temperature was 800 °C and the optimal Cu²⁺/Fe³⁺/starch ratio was 1:2:3.

Ansari et al. [46] received CoTiO₃/CoFe₂O₄ nanopowders with enhanced magnetic and electrochemical properties via the sol–gel autocombustion method. They compared fuel, capping, and the reducing properties of glucose, maltose, and starch. The effect of added sugars, Ti-to-Co ratio, and calcination temperature was also studied. Gelation and autocombustion temperatures were 60 °C and 120 °C, respectively, whereas the calcination process was carried out at two temperatures (650 and 750 °C) and lasted for 1 h. It was proved that the more complex sugar used in the synthesis as a reducing agent, the better-formed powder grains can be obtained. Using glucose, a highly agglomerated product was obtained, whereas for maltose and starch aggregates, spherical nanoparticles were obtained. They also confirmed the fact that the more hydroxyl groups are in sugar, the better the powder grains are covered with carbohydrate and no large agglomerates are formed, and in the later stages, spherical powder particles are formed as a result of the combustion process. It is also easier to control the size of nanoparticles. When the calcination temperature was lower (more fuel in the system), the reaction rate also slowed down (particles had more time to react), which resulted in a more regular grain shape. It was also found that by reducing the amount of starch in the system, the grain size and degree of agglomeration increase.

Smirnova et al. [47] used a mixture of glycine and starch to synthesize Mg(Fe_0.8Ga_0.2)₂O₄ spinel nanopowders. The use of glycine alone causes contamination of the powder by dimensional inhomogeneity of particles and larger crystallites are formed (50–100 nm), whereas for the mass ratio of glycine to starch 8:1, pure Mg(Fe_0.8Ga_0.2)₂O₄ crystallites of 40–50 nm are formed.

In turn, Agilandeswari and Kumar [48], using starch-assisted sol–gel combustion method, obtained Ca₃Co₄O₉ powder with improved magnetic and electrical properties. As a sol–gel mixture, solution containing cobalt and calcium nitrates, and starch was used. By heating such a mixture to 120 °C, a gel, which after further thermal treatment converted into a noncarbonate Ca₃Co₄O₉ phase, was obtained. On the basis of TG/DTA analysis, it was found that the starch presence contributed to a significant reduction in the reaction exothermicity, i.e., the exothermic oxidation of carbon generated a thermal reaction that allowed the conversion of cobalt oxide and calcium carbonate into calcium cobaltite.

Freitas et al. [49] compared four complexing agents: citric acid, glycine, corn starch, and gelatin in sol–gel autocombustion synthesis of LiCoO₂ nanopowders. SEM and XRD research has shown the rhombohedral phase in all samples and a small amount of cubic Co₃O₄ spinel in the samples with starch and gelatin, as well as in the blank test (without any gelation additive). Gelation temperature of each sample varied between 70 and 80 °C, but the significant dissimilarity was in the gelation time. The longest time was for the samples based on citric acid (5 h), next gelatin and glycine (3 h), and the shortest time was for the samples based on starch (1 h). Combustion temperature was 300 °C and calcination temperature was 700 °C. Based on the XRD data, the calculated average size of LiCoO₂ crystallites was as follows: 66–70 nm for glycine, 69–72 nm for citric acid, 70–75 nm for starch, and 73–75 nm for gelatin. The larger size of crystallites is related to the fuel type, because decomposition of starch and gelatin gives more energy to the system during the formation of nanocrystallites.

Motevalian and Salem [50] investigated the influence of glycine–potato starch mixing ratio on the structural properties of MgAl₂O₄ nanopowders synthesized with the use of sol–gel autocombustion method. Both additives are complexing agents as well as fuel additives. But starch changes ignition from flame to smoldering and because of that, it lowers the temperature of the autoignition process. Addition of starch counteracts agglomeration and facilitates the reaction between solid-phase and gas species, resulting in more pure spinel powder (purity increased after 10 wt% addition of starch).

Ramasami et al. [51] obtained photocatalytically and antimicrobially active ZnO powders via the sol–gel autocombustion process. As the carbohydrate additive, they have used tapioca starch pearls that they mixed with zinc nitrate hexahydrate in the mass ratio of nitrate:starch 1:1, 1:0.5, and 1:0.25. The best ratio was the middle one. Thanks to it, spherical nanoparticles of 45 nm were obtained.

Starch can also be a source of carbon in the synthesis of ceramic powders. Okano and Suzuki [52] developed a hydrothermal synthesis of carbonate spheres (CSs) using corn, potato, and rice starch. After hydrothermal treatment of aqueous starch suspensions at 160 °C for 4–16 h, followed by washing and drying, CSs were obtained. Chemically, these spheres did not differ from each other. As an effect of increasing starting mass and hydrothermal treatment time, larger particles were obtained. In the case of potato starch, probably due to its type B structure, the narrowest particles were obtained. This confirms the assumption that the initial crystalline structure of the starch interacts with hydrothermal reactions.

Wei et al. [53] obtained chromium oxide green pigment during the hydrothermal reaction in an autoclave (200 °C, 2.5 h) between Na₂CrO₄ and starch, and further calcination of amorphous Cr(OH)₃ (950 °C, 2 h). The mineralizer effect (carbonates and urea) on the synthesis conditions of the pigment was also studied. The results indicate that conversion of chromate was promoted by the type of mineralizer and its content. The best results (98.9% of conversion) were obtained in the presence of NaHCO₃ at 10–100 g/L, and when the temperature of the reaction was higher than 100 °C.

In turn, Yang et al. [54], in the single-stage carbonation-reduction process, where soluble starch served as a carbon carrier, received WC–Co composite nanopowders with spherical shape and average particle size in the range of 43–339 nm.

Visinescu et al. [55] obtained nanograins of Co_xZn_1−xAl₂O₄ (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1) blue pigment, using starch gelation process. In the first step, an aqueous solution containing starch and Zn (II), Co (II), and Al (III) salts was heated to 80 °C to obtain a gel. Then, the material was subjected to heat treatment at 800 °C to obtain a gel with high homogeneity of metallic precursors, which lowered the temperature required to spinel crystallization (from 713 to 684 °C).

As one can see, in combined methods, thanks to a synergy effect, starch allows to obtain ceramic powders with better properties and to lower the process temperature. Almost all types of ceramic powders (Table 3) can be obtained using combined methods, in particular, ferrites and spinels. In favor of starch is the fact that it gels faster than other organic agents. Due to the presence of a large number of hydroxyl groups in its structure, it is able to “stick” more and tightly to the grains of the resulting powder, thanks to which a desirable shape of particles, generally spherical, with a low degree of agglomeration, is obtained. By controlling its quantity, grain size of the ceramic powders can also be controlled. However, it is important to choose a proper fuel-to-oxidation ratio and not to overdo the starch content because we will get an impure final powder product.