In this study, proanthocyanidins-functionalized gold nanoparticles were synthesized via a hydrothermal method. UV-Vis and FTIR results indicated that the obtained products were mainly spherical in shape, and that the phenolic hydroxyl of proanthocyanidins had strong interactions with the gold surface. TEM and XRD determination revealed that the synthesized gold nanoparticles had a highly crystalline structure and good monodispersity.
The primary results indicate that proanthocyanidins-functionalized gold nanoparticles had high removal rates for the heavy metal ions and dye, which implies that they have potential applications as a new kind of adsorbent for the removal of contaminants in aqueous solution. Gold nanoparticles are one of the most extensively studied noble metal nanomaterials due to its potential applications in catalysis, optical and electronic devices, biodiagnostics and medicine [ 1 , 2 , 3 ].
Various wet chemical methods have been reported for the synthesis of gold nanoparticles, such as citrate reduction and the Brust-Schiffrin method [ 1 , 4 ]. It has been discovered that the capping agents or the shells of gold nanoparticles are very important for their physicochemical properties, toxicity and applications [ 2 , 5 , 6 ]. In addition, the development of novel synthetic strategies for the preparation of monodisperse gold nanoparticles and the assembly of nanoparticles into one- and two-dimensional structures are critical for their applications [ 7 , 8 ].
Recently, there has been a renewed interest in green synthesis of nanoparticles [ 9 , 10 , 11 , 12 , 13 , 14 ]. For example, gold nanomaterials were synthesized by using plant extracts as both the reducing and capping agents [ 15 ]. Plant extracts that have great potential in heavy metal accumulation and detoxification are the best candidates for nanoparticle green synthesis and environmental remediation applications.
Pomegranates are cultivated and consumed in large quantities in China. Pomegranate peels Punica granatum are usually discarded as waste. However, it has been found that pomegranate peel contains a significant portion of polyphenols and proanthocyanidins, and has the highest antioxidant activity among the peel, pulp and seed fractions [ 16 , 17 ].
Polyphenols and proanthocyanidins are well known for their antioxidant activity [ 19 , 20 , 21 ]. Therefore, the water extract of pomegranate peels could be a potential reducing compound used for the green synthesis of gold nanoparticles. Green synthesis of metal nanoparticles using plant extracts can minimize their toxicity, whereas most chemical methods for the synthesis by using hazardous compounds such as hydrazine, sodium borohydride, and dimethyl formamide DMF as reducing agents results in a more complicated and time-consuming process for removing the toxic compounds for biomedical applications.
Furthermore, synthesis of metal nanoparticles using plant extracts is very cost effective, and therefore can be used as an economical and valuable alternative to the large-scale production of metal nanoparticles. In addition, the full utilization of plant waste is a sustainable path for development. In this study, we explored using the water extract of pomegranate peels as both a reducing and a capping agent to prepare gold nanoparticles via a hydrothermal method. Since proanthocyanidins are the main components of the water extract of pomegranate peels, which are the mixtures of the several different compounds nearly one hundred phenolic compounds [ 17 ], we further focused on the use of proanthocyanidins for the synthesis of gold nanoparticles.
Proanthocyanidins, naturally occurring antioxidants widely available in vegetables, fruits, nuts, seeds, flowers, and bark, have been reported to possess a broad spectrum of biological, pharmacological, and therapeutic activities against free radicals and oxidative stress [ 19 , 22 , 23 , 24 ]. To the best of our knowledge, the green synthesis of proanthocyanidin-functionalized gold nanoparticles has not been thoroughly explored.
Proanthocyanidins were purchased from the Aladdin Reagent Co. Shanghai, China. All other reagents were of analytical purity grade.
The dried pomegranate peel was ground into a fine powder and collected through an mesh sieve. Pomegranate peel powder was degreased with petroleum ether solid-liquid ratio for 1 h, and then filtrated and dried. A microwave-assisted extraction method was used to extract the pomegranate peel powder. In a conical flask, 2 g of dried peel powder was added to 50 mL of pure water. The mixed solution was then centrifuged at 10, rpm for 10 min. The obtained supernatant was used for the following synthesis.
The supernatants were diluted by a factor of 40 with pure water. The mixed solution was stirred for 30 min. In a typical experiment, 50 mL of the 0. A dark red solution was obtained, which indicated that proanthocyanidin-functionalized gold nanoparticles had been produced. The pH of the final gold colloid was approximately 2. To explore the removal effect on heavy metal ions and methylene blue MB , gold nanoparticles were separated from the initial solution by centrifugation.
They were obtained by centrifugation at relative centrifugal force RCF 22, g for 15 min. The mixed solution was shaken at rpm for a predefined time generally 6 h. Gold nanoparticles were then centrifuged at RCF 22, g for 6 min, and an aliquot of the supernatant was used to determine the remaining concentration of the heavy metal ions by flame atomic absorption spectrometry. Based on the obtained concentrations of the heavy metal ions in the supernatant, the removal rate for heavy metal ions by proanthocyanidin-functionalized gold nanoparticles could be calculated.
The removal efficiency for heavy metal ions by gold nanoparticles under different pH conditions was also investigated. The sample was prepared by mixing the precipitate of the gold nanoparticles derived from the centrifugation of the gold colloid solution with a small amount of solid KBr. FTIR spectra of the pure proanthocyanidins was also measured for comparison. The sample for XRD determination was derived from centrifugation of the gold colloid solution.
The metal content was determined by flame atomic absorption spectroscopy equipped with a hollow cathode lamp and an air-acetylene flame AnalytikJena AG, novAA , Jena, Germany.
The wavelengths nm used for the determination of the analyses were: copper Figure 1 a shows the UV-Vis absorption spectra of the water extract of the pomegranate peels and the synthesized gold colloid in a typical experiment. The water extract of the pomegranate peels had no obvious UV absorption from nm to nm, as indicated in Figure 1 a.
There is an absorption band peaking at nm after the hydrothermal process, which can be ascribed to the plasmon resonance band PRB of the gold nanoparticles [ 25 ]. The representative TEM image of the obtained gold nanoparticles is shown in Figure 1 b. The size distribution of the gold nanoparticles is indicated in the inset. The average diameter of gold nanoparticles was As shown in Figure 1 b, the size distribution of the gold nanoparticles is relatively broad, wherein there are both some large and some small nanoparticles. In order to obtain monodispersed gold nanoparticles, pure proanthocyanidins were investigated in the following synthesis, since proanthocyanidins are the main components of the water extract of pomegranate peels.
Figure 2 a shows the UV-Vis absorption spectrum of the obtained proanthocyanidins-functionalized gold colloid in a typical experiment. The absorption band peaking at nm is the plasmon resonance band PRB of the gold nanoparticles, which is in accordance with the previous studies [ 25 ]. The single PRB also indicates that the obtained gold nanomaterials are mainly spherical in shape, in accordance with the Mie theory, and did not form agglomerates [ 26 , 27 ]. In most of the reported cases of green syntheses via plant extracts, the PRBs of the gold nanoparticles were usually broad and weak [ 21 , 28 ].
Here, the strong PRBs were due to the high productivity of gold nanoparticles in the synthesis. The stability of the gold colloid can be monitored by UV-Vis absorption spectra. The obtained gold colloid was very stable during storage, and could be maintained at room temperature for more than one month without obvious changes. In addition, the proanthocyanidin-functionalized gold nanoparticles showed good stability in the different NaCl concentrations 0.
The stability of the obtained gold nanoparticles was further verified in the following centrifugation-redispersion experiments. The proanthocyanidin-functionalized gold nanoparticles were collected by centrifugation, as indicated in the inset in Figure 2 a. They were able to be redispersed and recovered in pure water or a metal salts solution into a stable gold colloid.
Figure 2 b shows the UV-Vis absorption spectrum of proanthocyanidins-functionalized gold nanoparticles redispersed in the CuCl 2 solution. The UV-Vis absorption spectrum of the redispersed solution is nearly same as that of the initial solution in the synthesis. Gold nanoparticles can be further collected by centrifugation to test their adsorption ability for heavy metal ions as shown in inset in Figure 2 b.
It is known that the commonly used gold nanoparticles reduced by citrate or NaBH 4 method tend to aggregate in the repeat centrifugation-redispersion experiments. The extra stability of proanthocyanidins-functionalized gold nanoparticles in this study implied that the surfaces of the gold nanoparticles were well protected by the associated proanthocyanidins. Figure 3 shows the FTIR spectrum of the pure proanthocyanidins and proanthocyanidins-functionalized gold nanoparticles. These results confirmed that proanthocyanidins were associated with gold nanoparticles. The shift effect at the hydrogen bond region implied that phenolic hydroxyl of the proanthocyanidins had strong interactions with the surface of the gold nanoparticles.
FTIR absorption spectra of pure proanthocyanidins and the proanthocyanidin-functionalized gold nanoparticles. Figure 4 a shows the representative TEM image of the gold nanoparticles. Most of the gold nanoparticles were of spherical shape with a narrow size distribution. It has been noticed that some gold nanoparticles shared their proanthocyanidin shell, and formed a structure like a plum pudding model, as indicated in Figure 4 a.
The average diameter of the gold nanoparticles is Gold nanoparticles with different size can be obtained by using a seed growth method. The average size of the obtained gold nanoparticle is 28 nm, which is almost two times the mean size of the seeds. A high resolution transmission electron microscopy HRTEM image of one typical spherical gold nanoparticle is shown in Figure 4 b.
The lattice fringe spacing indicated in the image is 0. The crystalline nature of the gold nanoparticles was further verified by the XRD measurement, as shown in Figure 5. The peaks were assigned to diffractions from the , , and planes of face centered cubic fcc gold JCPDS , respectively [ 27 ]. This result indicates that the nanoparticles are pure, well-crystallized gold crystals. The width of the peak was employed to calculate the average crystallite size using the Scherrer equation. The calculated average size is A similar difference between TEM and XRD determination of nanomaterials has also been reported in a recent study [ 31 ].
In this study, oligomeric proanthocyanidin complexes OPCs, dimeric, trimeric and tetrameric proanthocyanidin were used in the synthesis. OPCs are primarily known for their antioxidant activity against free radicals and oxidative stress [ 32 ]. Thus, proanthocyanidin can be considered as a class of reducing agents.
The reaction for the formation of gold nanoparticles is believed to be a typical redox process, wherein the gold ions are reduced to gold atoms by proanthocyanidins. The multiple hydroxyl groups —OH from the basic monomer catechin and epicatechin are the potential reactive sites for reducing Au III in the synthesis [ 21 ].
The initial step of oxidation may occur on the B-ring of the catechin due to partial deprotonation, which leads to the transformation of o -phenols to o -quinones, a fundamental step in browning. O -quinones are highly reactive species that form dimers and subsequently polymers due to prolonged autoxidation with other polyphenolic molecules [ 33 ]. Some additional hydroxyl groups from the catechin and epicatechin interacted with the gold surface, as revealed by FTIR determination, which was finally made up of the functionalized shell of the gold nanoparticles.
The discharge of various dyes into the hydrosphere is a significant source of water pollution, due to their recalcitrant nature. Methylene blue MB , a heterocyclic aromatic chemical compound, is extensively used in the textile industry to colorize products, which can be considered to be one kind of typical dye discharged into the environment [ 34 , 35 ]. MB is the most commonly used substance for dying cotton, wood and silk. Various materials, such as natural materials, industrial solid wastes, agricultural by-products, and biosorbents, have been developed in the removal of MB from wastewater [ 36 , 37 , 38 ].
In this study, the removal of MB in water solution by proanthocyanidins-functionalized gold nanoparticles was investigated. Citrate-gold nanoparticles were also tested as the control sample, as indicated in Figure 6. The solution color did not change with the addition of citrate-gold nanoparticles. In addition, it has been reported that the mixing of the citrate-gold nanoparticles with methylene blue MB enhanced the extinction coefficient and absorbance of the dye [ 39 , 40 ]. However, the blue solution was decolored by proanthocyanidin-functionalized gold nanoparticles within 30 min, as shown in Figure 6.
The adsorption mechanism is believed to be related with the interactions between proanthocyanidins on the gold nanoparticle surface and molecular moiety of MB. Methylene blue molecules are likely attracted to the nanoparticle surface by dipole-dipole interactions between the nitrogen in the methylene blue and the phenolic groups of proanthocyanidins and their chemical association. In addition to the dyes, we further explored proanthocyanidin-functionalized gold nanoparticles for the removal of heavy metal ions in aqueous solutions. Toxic heavy metal ions have been excessively released into the aquatic environment due to various industrial activities, and have created a major global concern [ 41 ].
The major toxic metal ions in water that are hazardous to humans, as well as other forms of life, are cadmium, copper, nickel, lead, mercury and chromium. However, these conventional techniques have their own inherent limitations. It is, therefore, necessary to develop new methods or nanomaterials for the low-cost, high-efficiency minimization of chemical or biological sludge and the regeneration of biosorbents.
Gold nanoparticles have been used for a variety of applications, such as in catalysis, optical and electronic devices, biodiagnostics and bioimaging [ 2 , 43 ]. However, there are few studies on the application of gold nanoparticles for the removal of heavy metal ions. The gold nanoparticles in this study were functionalized by oligomeric proanthocyanidin complexes, which are also very potent metal-chelating agents derived from the multiple hydroxyl groups —OH of catechin and epicatechin in proanthocyanidin [ 22 , 33 ].
The removal efficiency of proanthocyanidins-functionalized gold nanoparticles for heavy metal ions under different conditions was evaluated. It was found that the pH and adsorption time are the two critical factors influencing the removal efficiency for heavy metal ions. The results indicated that the removal efficiency increased as pH rose from 4.
With a pH of 8. Since a pH of 8. The results revealed that the adsorption process was very quick at the initial stage, gradually slowing down after 30 min incubation, and was then constant from 3 h to 6 h. One advantage of proanthocyanidin-functionalized gold nanoparticles in removing heavy metal ions is that gold nanoparticles are easy to collect from the aqueous solution by centrifugation, as indicated in Figure 2 b, and then for recovery for further utilization.
Nearly all of the gold nanoparticles were able to be collected from the metal salts solution, as judged by the solution color and UV absorption spectra. The superior stability of proanthocyanidin-functionalized gold nanoparticles in repeat centrifugation-redispersion experiments implied that the surface of gold nanoparticles was well protected by the associated proanthocyanidins. In addition, some gold nanoparticles are able to share their proanthocyanidins shell, as indicated in Figure 4 a, and form a plum pudding model structure, which may contribute to their characteristic stability and easy manipulation.
The adsorption of heavy metal ions by proanthocyanidin-functionalized gold nanoparticles can be ascribed to the proanthocyanidin shell, since the PRB band of gold nanoparticles did not show obvious changes when they were redispersed in the heavy metal ion solution. The interaction between metal ions and the remaining hydroxyl groups —OH of proanthocyanidin on the gold surface plays an important role in sequestering the metal ions.
The adsorption process can be schematically illustrated in Figure 8. Oligomeric proanthocyanidin complexes were well covered on the gold nanoparticle surface, the remaining hydroxyl groups —OH of catechin and epicatechin in proanthocyanidin interacted with the metal ions and acted as a bidentate ligand to coordinate with the heavy metal ions.
For example, recent spectroscopic determinations have confirmed that the Cu II -catechin complex formation in which Cu II ions can react with catechin in the presence of oxygen in a metal chelation and the oxidation process [ 22 ]. Recently, Zero valent iron nanoparticles nZVI have been explored for the removal of heavy metal ions in groundwater and wastewater [ 44 ]. The mechanisms that facilitate the removal are complex.
It has been speculated that the core-shell structure in the iron nanoparticles involve the removal processes wherein the metallic iron core acts as an electron donor source, promoting reduction of the compounds. A variety of heavy metal ions can be removed by nZVI with high rates and capacities [ 44 , 45 ]. However, there are some disadvantages to using nZVI, such as the fact that some of the metal ions can be released from the nZVI over long periods of time [ 44 ].
Some structural parameters were calculated and summarized in Table 3 with good matching with theoretical data. Structural parameters calculated from XRD data [ 89 ]. SEM and TEM can be used to study the morphology and surface characteristics of the perovskite nanomaterials.
The SEM images for the different perovskites were shown in Figure 4 presenting different morphologies depending on the kind of metal ion at B-site, respectively. LaFeO 3 showed dissimilar morphology with a porous surface containing particles with bonelike shape. In addition, LaFeO 3 presented greater electrocatalytic activity toward hydrogen evolution reaction compared to other types of perovskites [ 90 ].
Furthermore, the high-resolution TEM HRTEM can be used to show the different morphologies and particle characteristics of the different perovskites [ 86 , 88 , 95 ]. The electrochemical performance and electrocatalytic activity of the perovskites are greatly associated with the specific surface area of the materials; therefore, it is necessary to measure the specific surface area of the prepared materials.
The preparation conditions, synthesis method, type of A- and B-site metals, and presence of different dopants can greatly affect the surface area of the prepared perovskites [ 86 , 87 , 95 ]. Biniwale et al. The order of decreasing the surface area of the prepared LaFeO 3 was sol-gel Sol-gel and combustion methods resulted in porous surface with internal pores contributing to higher surface area, while coprecipitation method resulted in less internal pores and lower surface area due to longer calcination time [ 87 ].
Thermal analysis can be utilized to identify the thermal stability and the decomposition temperature of the prepared perovskites. The optimum calcination temperature of any perovskite can be identified using thermal analysis [ 88 , 96 , 97 ]. The chemical bonding and chemical structure of the prepared perovskites can be investigated via FTIR. Another band appeared at cm —1 was related to the deformation vibration mode of O-Fe-O. LaFeO 3 prepared via coprecipitation method showed a sharp band at cm —1 , which is related to La-O in lanthanum oxide.
In case of the other two methods, the band at cm —1 disappeared indicating the formation of relatively pure perovskite phase. Other bands appeared at and cm —1 , indicating other phases in case of coprecipitation method. As a result and as mentioned in literature, the absorption peak around cm —1 was related to the stretching modes of metallic oxygen bond [ 87 , 97 — ].
The surface compositions of the various components of the prepared perovskites can be identified via XPS [ — ]. Lee et al. The O 1s binding energy values at The peaks at The chemisorbed oxygen species appeared at binding energy higher than that of lattice oxygen species by 2. The peak appeared at Inorganic perovskite-type oxides exhibited attractive physical and chemical characteristics such as electronic conductivity, electrically active structure, the oxide ions mobility through the crystal lattice, variations on the content of the oxygen, thermal and chemical stability, and supermagnetic, photocatalytic, thermoelectric, and dielectric properties.
They are fascinating nanomaterials for wide applications in catalysis, fuel cells, and electrochemical sensing.
Nanoperovskites are recently utilized in electrochemical sensing of alcohols, gases, amino acids, acetone, glucose, H 2 O 2 , and neurotransmitters exhibiting good selectivity, sensitivity, unique long-term stability, excellent reproducibility, and anti-interference ability. Moreover, they have been utilized as catalysts in oxygen reduction and hydrogen evolution reactions exhibiting high electrocatalytic activity, lower activation energy, and high electron transfer kinetics.
In addition, some perovskites are promising candidates for the development of effective anodic catalysts for direct fuel cells showing high catalytic performance. There are a number of requirements that the materials utilized as gas sensors must satisfy, namely, good resemblance with the target gases, manufacturability, hydrothermal stability, convenient electronic structure, resistance to poisoning, and adaptation with existing technologies.
Perovskites including titanates, ferrites, and cobaltates were utilized as gas sensors for detecting CO, NO 2 , methanol, ethanol, and hydrocarbons [ — ]. LaCoO 3 prepared via high-energy ball milling exhibited the highest amount of grain boundaries, the best CO gas sensing properties, and the smallest crystallite size of 11 nm compared to that prepared via solid-state and sol-gel reactions.
In addition, the specific surface area increased greatly from 4 m 2 g —1 to 66 m 2 g —1 by extra milling step, and the mobility of the oxygen was enhanced by growing the extra milling step and surface area [ ]. A summary of various perovskite oxides for different gas sensing was given in Table 4. It is very important to analytically determine H 2 O 2 and glucose in many fields like food, clinic, and pharmaceutical analyses.
H 2 O 2 is considered one of the most important oxidizing agents in chemical and food industries. Glucose is a fundamental metabolite for most of the living organisms and for the clinical examination of diabetes mellitus, a worldwide health problem. As a result, it is very important to construct biosensors for the sensitive determination of H 2 O 2 and glucose [ — ].
Different types of enzymatic glucose sensors were constructed and used in the literature exhibiting the advantages of simplicity and sensitivity. However, enzymatic glucose sensors suffered from the lack of stability and the difficult procedures required for the effective immobilization of enzyme on the electrode surface. The lack of enzyme stability was attributed to its intrinsic nature because the enzyme activity was highly affected by poisonous chemicals, pH, temperature, humidity, etc. As a result, most attention was given for sensitive, simple, stable, and selective nonenzymatic glucose sensor.
Different novel materials were proposed for the electrocatalytic oxidation of glucose like noble nanometals, nanoalloys, metal oxides, and inorganic perovskite oxides. Inorganic perovskite oxides as nanomaterials exhibited fascinating properties for glucose sensing like ferroelectricity, superconductivity, charge ordering, high thermopower, good biocompatibility, catalytic activity, and the ability of the perovskite structure to accommodate different metallic ions [ — ]. Zhen Zhang et al. This sensor displayed perfect electrocatalytic activity toward H 2 O 2 and glucose oxidation in alkaline medium due to the presence of large amount of active sites in the modifier.
The linear dynamic range for H 2 O 2 at this surface was 0. For glucose, two ranges were obtained from 0. The proposed sensor exhibited rapid response, excellent long-term stability, and anti-interference ability toward ascorbic acid, uric acid, and dopamine [ ]. Furthermore, Atta et al. The studied sensor exhibited high electrocatalytic activity toward glucose oxidation exploring the effective synergism between SrPdO 3 and gold nanoparticles.
SrPdO 3 perovskite facilitated the charge transfer process and acted as an effective supporting substrate for gold nanoparticles. The catalytic activity of SrPdO 3 was attributed to the deficiency of the surface for oxygen which resulted in enhanced intrinsic reactivity toward glucose oxidation. This nanocomposite showed good performance toward glucose sensing in terms of highly reproducible response, high sensitivity, wide linearity, low detection limit, good selectivity, long-term stability, and applicability in real urine samples and blood serum [ ].
A summary of different types of perovskites used for enzymatic and nonenzymatic H 2 O 2 and glucose sensing was given in Table 5 , exhibiting high sensitivity, wide linear range, low detection limit, anti-interference ability, applicability in real samples, and long-term stability. Dopamine DA is an essential catecholamine neurotransmitter that exists in the mammalian central nervous system. Therefore, it is very important to present a modified surface which can be sensitively and selectively detect DA even in presence of high concentration of AA and UA.
Atta et al. The proposed sensor showed high sensitivity, good selectivity, and anti-interference ability [ 89 ]. One characteristic of perovskite is the deficiency of its surface for oxygen. Furthermore, the descriptor that controls the catalytic process in perovskites is the type of transition metal in the perovskite, which is related to the number of occupied d orbital states of a specific symmetry, for example, of the active metal. This is associated with the surface ability to bond oxygen on the basis of the calculations of the density functional theory.
A summary of different perovskites used for neurotransmitters sensing was given in Table 6. Fuel cells have come into view as efficient alternatives to combustion engines due to their potential to minimize the environmental influence of the use of fossil fuels. A fuel cell uses certain type of chemical fuel as its energy source, and there is a direct transformation of chemical energy into electrical energy like a battery. Fuel cells are attractive because of their great efficiency, modular and distributed nature, low emissions, zero noise pollution, and role in any future hydrogen fuel economy.
There are several types of fuel cells depending on operating temperature, fuel type, electrolyte type, and mobile ions. Polymer electrolyte membrane fuel cells, molten carbonate, phosphoric acid or alkali fuel cells, and solid oxide fuel cell are the most common examples of fuel cells [ ].
Table 7 contained some fuel cells types and some selected features [ ]. Here we will concern on solid oxide fuel cells. Solid oxide fuel cells SOFCs , based on conducting electrolyte in the form of an oxide-ion, can generate electricity and heat and they are considered as energy-saving, environment-friendly, and effective energy conversion devices. SOFCs exhibited several features compared to the other types of fuel cells like high-energy conversion efficiency, cheap materials, low sensitivity to the fuel impurities, low pollution emissions, environmental compatibility, and excellent fuel flexibility [ — ].
Figure 8 showed the working principle of a solid oxide fuel cell [ ]. The high temperature of SOFC operation resulted in the difficult choice of the proper materials and the decreased cell durability. Perovskite oxides exhibited fascinating properties like good electrical conductivity similar to that of metals, high ionic conductivity, and perfect mixed ionic and electronic conductivity.
Depending on the differences in the electrical conductive characteristics of perovskites, they are chosen as an effective component in SOFC [ 9 ]. In addition, mixed-conduction perovskite oxides possess beneficial electrochemical reaction; structural, thermal, and chemical stabilities; high electrical conductivity; high catalytic activity toward the oxygen reduction; and ideal mixed electronic and ionic conductivities to be used as effective component for intermediate temperatures SOFC IT-SOFC [ — ].
Shao and Haile utilized Ba 0.
This anode material showed long-term stability, stability in reducing atmosphere, tolerance to sulfur, and characteristic of oxygen deficiency [ ]. Table 8 contained a summary of different perovskites used as anode and cathode for SOFC illustrating the fuel type, the operating temperature, and the maximum power density. Fuel cell types and selected features [ ]. Perovskite oxides can be widely used as catalyst in modern chemical industry, exhibiting appropriate solid-state, surface, and morphological properties [ 6 ].
Several perovskites exhibited enhanced catalytic activity toward different reactions like hydrogen evolution and oxygen evolution and reduction reactions [ 9 ]. Because of the advantages of high heat of combustion, abundant sources, and no pollution, hydrogen is considered as an ideal fuel.
Hydrogen evolution reaction HER is a fascinating reaction in the renewable energy field. This reaction is very important in i metal electrodeposition and corrosion in acids, ii storage of energy through production of hydrogen, and iii as the hydrogen oxidation reaction reverse in low-temperature fuel cells. One of the most studied reactions in electrochemistry is the electrocatalysis in HER. The material used for HER should have i intrinsic electrocatalytic activity, ii considerable active surface area per unit volume, and iii good stability.
To reduce the cost of electrolytic HER, the overpotential required for the operation of the electrolyzer at considerable current densities should be reduced. The overpotential reduction can be achieved through the electrode active surface area enhancement or by the selection of electrode materials of high catalytic activity. The steps of the reaction in acidic solutions are as follows:. The first step in HER is the proton discharge volume reaction, Eq. Furthermore, the order of decreasing the catalytic activity in case of doped samples was La 0. On the other hand, Galal et al. The rate-determining step was the hydrogen adsorption on the catalyst and the order of the reaction at the catalyst surface was 0.
Table 9 contained a summary of different perovskites used as catalysts for HER with the values of exchange current density at constant overpotential, activation energy, reaction order, and the rate-determining step. Oxygen reduction reaction ORR and oxygen evolution reaction OER are considered one of the most important electrode reactions in many industrial processes like fuel cells, metal electrowinning, water electrolysis, electro-organic synthesis, cathodic protection, and rechargeable metal air batteries [ 95 , — ].
Platinum-based catalysts and precious metal oxides are the most common catalysts for ORR or OER, but they are expensive and scarce. Mixed metal perovskite oxides of transition and rare earth metals are promising low-cost alternatives to precious metal catalysts for both ORR and OER [ ]. Perovskite oxide exhibited unique electronic and magnetic properties, defective structure, and good cation ordering resulting in disorder-free channels of oxygen vacancies and enhanced mobility of oxygen ions [ ].
Ruizhi Yang et al. The proposed catalyst exhibited higher catalytic activity toward OER than the unmodified electrode [ ]. Furthermore, Galal et al. LaFeO 3 exhibited greater electrocatalytic activity toward OER by about folds compared to the unmodified electrode. The current density at 1. This was attributed to the matrix effect induced by the stable crystal structure of the perovskite [ 95 ]. In addition, La 0. Solar energy is a green source of energy that can be used instead of energy sources based on fossil fuels. Solar radiation can be directly converted into electrical energy in a suitable way creating various applications for solar energy.
Summary. Intended as a reference for basic and practical knowledge about the synthesis, characterization, and applications of nanotechnology for students. Nanomaterial Synthesis, Characterization, and Application, Mahmood Ghoranneviss, Ajay Soni, Alireza Talebitaher, and Necdet Aslan Editorial (2 pages).
Solar energy can be efficiently converted into electricity using photovoltaic solar cells based on silicon. The obvious disadvantage of silicon-based solar cell is the high price of electricity generated from it so that there is a potential need to develop solar cell with low cost. A is usually divalent and B is tetravalent when O 2— anion is used. As indicated by the example, halide perovskites contain monovalent and divalent cations in A- and B-sites, respectively, to maintain electrical neutrality [ , ].
Like oxide perovskite, the tolerance factor of halide perovskite should be as close to one to maintain a stable and symmetrical crystal structure [ ]. The quality of the perovskite film is very crucial for solar cells. Several methods have been used to form perovskite films with high quality such as single step solution method, vapor assistant solution process, sequential deposition of inorganic and organic precursor, and coevaporation of the precursors [ ].
CH 3 NH 3 PbI 3 perovskite film was prepared with high quality by adding small amounts of N-methylpyrrolidone and a mixture of g-butyrolactone and dimethylsulfoxide via a solution method. A power conversion efficiency of Table 11 contained a summary of different models based on perovskites used for solar cells applications with the values of power conversion efficiency, fill factor, method of perovskite formation, solar cell composition, cost, and stability.
Inorganic perovskite-type oxides are excellent nanomaterials for wide applications in catalysis, fuel cells, and electrochemical sensing, exhibiting attractive physical and chemical characteristics. They showed electronic conductivity, electrically active structure, the oxide ions mobility through the crystal lattice, variations on the content of the oxygen, thermal and chemical stability and supermagnetic, photocatalytic, thermoelectric, and dielectric properties.
Nanoperovskites have been utilized as catalysts in oxygen reduction and hydrogen evolution reactions exhibiting high electrocatalytic activity, lower activation energy and high electron transfer kinetics. Moreover, they are recently utilized in electrochemical sensing of alcohols, gases, glucose, H 2 O 2 , and neurotransmitters. They can enhance the catalytic performance in terms of unique long-term stability, sensitivity, excellent reproducibility, selectivity, and anti-interference ability. In addition, organometallic halide perovskites exhibited efficient intrinsic properties to be utilized as a photovoltaic solar cell with good stability and high efficiency.
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Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Atta, Ahmed Galal and Ekram H. Downloaded: Abstract Inorganic perovskite-type oxides are fascinating nanomaterials for wide applications in catalysis, fuel cells, and electrochemical sensing. Introduction 1. General introduction to perovskites The mineral CaTiO 3 was discovered by Geologist Gustav Rose in the Ural Mountains in , and it was named perovskite in recognition beholden to Count Lev Alexevich von Perovski, an eminent Russian mineralogist [ 1 — 5 ].
Table 1. Crystallography of the perovskite structure In the ABO 3 form, B is a transition metal ion with small radius, larger A ion is an alkali earth metals or lanthanides with larger radius, and O is the oxygen ion with the ratio of Typical properties of perovskites Perovskite exhibited a variety of fascinating properties like ferroelectricity as in case of BaTiO 3 and superconductivity as in case of Ba 2 YCu 3 O 7.
Table 2. Dielectric properties There are some properties inherent to dielectric materials like ferroelectricity, piezoelectricity, electrostriction, and pyroelectricity. Electrical conductivity and superconductivity One of the obvious properties of perovskites is superconductivity. Catalytic activity Perovskites showed excellent catalytic activity and high chemical stability; therefore, they were studied in a wide range in the catalysis of different reactions.
Methods of perovskite synthesis 2. Solid-state reactions In solid-state reactions, the raw materials and the final products are in the solid-state therefore nitrates, carbonates, oxides, and others can be mixed with the stoichiometric ratios. Gas phase preparations Gas phase reaction or transport can be used for the deposition of perovskite films with a specific thickness and composition.
Wet chemical methods solution preparation These methods included the sol-gel preparation, coprecipitation of metal ions using precipitating agents like cyanide, oxalate, carbonate, citrate, hydroxide ions, etc. Precipitation 2. Oxalate-based preparation This method is built on the assimilation of oxalic acid with carbonates, hydroxides, or oxides producing metal oxalates, water, and carbon dioxide as products [ 22 ].
Hydroxide-based preparation This method is often used due to its low solubility and the possible variety of precipitation schemes. Acetate-based preparation Different perovskites were prepared by mixing acetate ions alone or together with nitrate ions with the metal ions salts.
Citrate-based preparation Citrate precursors can be used and undergo several decomposition steps in the synthesis of perovskite [ 29 ]. Cyanide-based preparation Rare earth orthoferrites REFeO 3 and cobalt compounds RECoO 3 were prepared using cyanides complexes via thermal decomposition of the rare earth ferricyanide and cobalticyanide compounds [ 30 ].
Thermal treatment 2. Freeze-drying The freeze-drying method can be achieved through the following steps: i dissolution of the starting salts in the suitable solvent, water in most cases; ii freezing the solution very fast to keep its chemical homogeneity; iii freeze-drying the frozen solution to get the dehydrated salts without passing through the liquid phase; and iv decomposition of the dehydrated salts to give the desired perovskite powder. Plasma spray-drying This method was applicable to various precursors, including gaseous, liquid, and solid materials.
Combustion A redox reaction, which is thermally induced, occurs between the oxidant and fuel. Doping of perovskites The different properties of perovskites and their catalytic activity are highly affected by the method of synthesis, conditions of calcination time, atmosphere, fuel, temperature, etc.
Characterization of perovskites X-ray powder diffraction XRD can be used to differentiate the different phases of the prepared perovskites. XRD XRD can be used for the phase identification and the relative percents of different phases of the prepared materials. Table 3. BET The electrochemical performance and electrocatalytic activity of the perovskites are greatly associated with the specific surface area of the materials; therefore, it is necessary to measure the specific surface area of the prepared materials.
Thermal analysis Thermal analysis can be utilized to identify the thermal stability and the decomposition temperature of the prepared perovskites. XPS The surface compositions of the various components of the prepared perovskites can be identified via XPS [ — ]. Applications of perovskites Inorganic perovskite-type oxides exhibited attractive physical and chemical characteristics such as electronic conductivity, electrically active structure, the oxide ions mobility through the crystal lattice, variations on the content of the oxygen, thermal and chemical stability, and supermagnetic, photocatalytic, thermoelectric, and dielectric properties.
Sensors and biosensors 5. Gas sensors There are a number of requirements that the materials utilized as gas sensors must satisfy, namely, good resemblance with the target gases, manufacturability, hydrothermal stability, convenient electronic structure, resistance to poisoning, and adaptation with existing technologies.
Table 4. A summary of different perovskites for gas sensing. Glucose sensor It is very important to analytically determine H 2 O 2 and glucose in many fields like food, clinic, and pharmaceutical analyses. Table 5. A summary of different perovskites for H 2 O 2 and glucose sensing. Neurotransmitters sensor Dopamine DA is an essential catecholamine neurotransmitter that exists in the mammalian central nervous system. Table 6. A summary of different perovskites for neurotransmitters sensing.
Solid oxide fuel cells Fuel cells have come into view as efficient alternatives to combustion engines due to their potential to minimize the environmental influence of the use of fossil fuels. Table 7. Table 8. A summary of different perovskites for SOFC.
Catalyst Perovskite oxides can be widely used as catalyst in modern chemical industry, exhibiting appropriate solid-state, surface, and morphological properties [ 6 ]. Hydrogen evolution reaction Because of the advantages of high heat of combustion, abundant sources, and no pollution, hydrogen is considered as an ideal fuel. Table 9. A summary of different perovskites for HER catalysis. Oxygen reduction and oxygen evolution reactions Oxygen reduction reaction ORR and oxygen evolution reaction OER are considered one of the most important electrode reactions in many industrial processes like fuel cells, metal electrowinning, water electrolysis, electro-organic synthesis, cathodic protection, and rechargeable metal air batteries [ 95 , — ].
Perovskite Medium Exchange current density mA. Table Solar cells Solar energy is a green source of energy that can be used instead of energy sources based on fossil fuels. A summary of different models based perovskites for Solar cells applications. Conclusions Inorganic perovskite-type oxides are excellent nanomaterials for wide applications in catalysis, fuel cells, and electrochemical sensing, exhibiting attractive physical and chemical characteristics. Acknowledgments The authors would like to acknowledge the financial support from Cairo University through the Vice President Office for Research Funds.
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More statistics for editors and authors Login to your personal dashboard for more detailed statistics on your publications. Access personal reporting. More About Us. SrTiO 3 n-type. ReO 3. PbCrO 3. NaTaO 3. NaWO 3. LaCrO 3. SrTiO 3. KMoO 3. CaMnO 3. BaTiO 3. SrNbO 3. LaMnO 3. KTaO 3.