Multidisciplinary methods (co-precipitation, ultrasonic, microwave, reflux and hydrothermal) for synthesis and characterization of CaMn3O6 nanostructures and its photocatalytic water splitting performance

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Multidisciplinary methods (co-precipitation, ultrasonic, microwave, reflux and hydrothermal) for synthesis and characterization of CaMn3O6 nanostructures and its photocatalytic water splitting performance Sousan Gholamrezaei a, Maryam Ghiyasiyan-Arani a, Masoud Salavati-Niasari a,*, Hossein Moayedi b,c,** a

Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P. O. Box. 87317e51167, Islamic Republic of Iran b Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Viet Nam c Faculty of Civil Engineering, Ton Duc Thang University, Ho Chi Minh City, Viet Nam

highlights
Multidisciplinary techniques based wet-chemistry method utilize for production of CaMn3O6 nanocatalysts.
Various preparation techniques foster particles formation with different morphologies.
CaMn3O6 nanocatalysts well growth via co-precipitation technique.
Unique morphologies, properties and crystalline texture support its catalytic activities.
Presence of redox species like Mn enhance O2 evolution performances.

article info

abstract

Article history:

Production of hydrogen and oxygen from water splitting reaction under visible light is a

Received 7 June 2019

simple method for conversion of solar-to-hydrogen energy and it is a hopeful clean and

Received in revised form

renewable method for H2 fuel generation. However, there is still a lack of potential ma-

22 July 2019

terials with significant activity under visible light. Because of safety, chemical inertness,

Accepted 16 August 2019

low cost, stability and other characteristics, transition metal oxide semiconductors have

Available online 9 September 2019

been widely applied as photocatalysts for hydrogen generation. Albeit, wide usage of semiconductor photocatalysts were prevented by its inability to exploit solar energy of

Keywords:

visible region. Here we show synthesis of a nano-sized mixed metal oxide (MMO) Ca3MnO6

Nanostructures

through wet-chemistry methods such as co-precipitation, ultrasonic, microwave, reflux,

Ceramic

and hydrothermal methods. The nano-sized Ca3MnO6 has initially selected based on

Ultrasonic

morphology and respective particle diameters. The selected sample shows a well-defined

Water splitting

single crystal, free from any impurities, complete structural formation, and a band gap

O2 evolution

energy (Eg) of around 5.3 eV. The best product synthesized in ultrasonic method which

* Corresponding author. ** Corresponding author. Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Viet Nam. E-mail addresses: [email protected] (M. Salavati-Niasari), [email protected] (H. Moayedi). https://doi.org/10.1016/j.ijhydene.2019.08.141 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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shows the best morphology, purity and the highest efficiency for splitting of water to hydrogen and oxygen. Irrespective of preparation methods and morphologies, all samples split water into hydrogen and oxygen, as confirmed from their respective photocatalytic analysis. When the selected sample combined with (NH4)2Ce(NO3)6, the single-crystal Ca3MnO6 nanoparticles split water into hydrogen and oxygen more efficiently under visible light. Our findings demonstrate the importance of nanostructured Ca3MnO6 singlecrystal photocatalysts in solar water splitting. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Huge energy generation/storage are main key challenges in the near future, with emphasizing their clean, safe and sustainable sources. Hydrogen is an important carrier of energy with no emission of pollutants when burned. Water splitting, over the past decade, has been achieved a great interest in energy systems. It is a pathway of water dissociation into oxygen (O2) and hydrogen (H2) Eq. (1): 2H2O / 2H2 þ O2

(1)

Photocatalytic [1], photoelectrochemical [2], radiolysis [3], thermal decomposition [4], and photobiological [5] methods are promoted extensively for water splitting technologies. Photocatalytic water splitting is one of the major techniques which operates in a photosynthetic cell [6]. In this cell, there are two redox systems; one reacts with the outer electrons in the photocathode and the other reacts with the inner electrons at the counter electrode [7]. The electrochemical setup works in acidic, basic, or neutral media. The neutral medium is the most challenging condition as its kinetic is slowed for water dissociation [8]. Among different elements, Mn is an attractive element for water splitting due to the suitable properties such as earth abundance, environmental friendly and low cost [7e9]. Mnbased compounds as an efficient catalyst used in nature for oxidizing water on plants, cyanobacteria, and algae that is called water-oxidizing complex (WOC) [10]. Semiconducting nanomaterials based mixed metal oxides (MMOs) are recently used as photocatalysts and photoelectrode materials in advanced solar water splitting systems [9]. Owing to the structure of nanosized MMOs, efficient photoexcitation can propose under solar illumination. In the MMO nanostructures, their catalytic activity, selectivity, conductivity, and photonic efficiency are closely connected with the specific surface area (SSA) [11]. Superior activated surface in the semiconducting MMOs can facilitate both charge separation and stability of the materials under photoexcitation [12]. Evaluation of the photocatalytic and photoelectrochemical activities of MMOs for water splitting are at a crucial stage in enabling an effective system [2]. Previous approaches to improve materials for water dissociation have focused on elemental properties, surface texturing or composite fabrication by incorporation of additional water dissociation compounds [13]. Mixed metal oxides

(MMOs), owing to their structural defects and morphology, can create reactive sites - such as corners, edges, and grain boundaries - that can further affect the rate of the water splitting. The MMOs structures involve abundant and inexpensive elements are predictable for successful water dissociation reaction [14]. Among various MMOs that utilized in photocatalytic water splitting, manganese (Mn) based MMOs can be a potential candidate, owing to their inherent catalytic and electrochemical properties. Mn is a chemically active element which can easily oxidized water and react with it. The introduction of Mn3þ into the lattice structure of MMOs can induce charge transfer following the Mn3þ / Mn4þ redox reaction [15]. The compounds in CaxMnyOz system can represent as suitable candidate for water splitting. As mentioned before, the flexibility in oxidation state of the Mn ion allows a rich profile in energy storage and production [16]. Various structures of Ca e Mn e O have been proposed such as CaMnO3 [17], CaMn2O4 [18], and CaMn4O8 [19]. CaMn3O6 is unique among manganese based MMOs and description of its physicochemical properties is therefore incomplete [20,21]. Here we demonstrate, for the first time, multidisciplinary methods for preparation of CaMn3O6 nanoparticles. Furthermore, the optimized sample selected for study the water splitting photocatalytic activities in neutral media. Specifically, the effect of oxidant on water splitting catalysts investigated in details.

Experimental Materials In the current study, all chemical reagents, MnCl2.4H2O (197.91 g/mol), CaCl2.2H2O (Mw ¼ 147.01 g/mol), KMnO4 (Mw ¼ 158.03 g/mol), were purchased from Merck-Germany and used without further purification. These starting cationic sources were used in order to prepare CaMn3O6 nanoparticles.

Synthesis of CaMn3O6 It is renowned that the mixed metal oxides can be obtained by two main methods; either direct reaction of two or more solids, or linking of polyhedral building units from solution and deposition of the newly formed solid.

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In the current study, a series of solution techniques were utilized such as co-precipitation, hydrothermal, reflux, microwave, and ultrasonic methods in order to synthesize pure CaMn3O6 nanoparticles. All reaction conditions are tabulated in Table 1. Co-precipitation method e Co-precipitation is one of the solution techniques for preparation of nanoparticles, in which the primary cationic sources are introduced into alkaline medium with a homogeneous distribution [22]. In contrast to other techniques, the co-precipitation technique does not require costly equipment, stringent reaction conditions, or complex procedures. In this approach, 0.12 ± 0.001 g KMnO4 was dissolved in 10 ml distilled water to obtain a purple and clear solution. This solution was transferred into an ice bath, where 5.0 ± 0.01 g KOH was added to the solution, and stirred for 15 min (container 1). In the second container, 0.58 ± 0.003 g CaCl2 and 0.25 ± 0.004 g MnCl2 were dissolved in 3 ml distilled water. The contents of second container were added dropwise into the alkaline solution, where dark-brown precipitates was separated from the bulk solution. In order to terminate the reaction profile and neutralized the contents, 100 ml of distilled water was added to above admixture and stirred for 2 h, and finally filtered using a Bu¨chner funnel. The resulting precipitates were washed several times with water and ethanol and dried in an oven at 80  C for 24 h. The obtaining dark-gray powders were calcined at 400  C for 4 h. Hydrothermal method - Solution methods are also extended using hydrothermal techniques, where, in a sealed vessel, the reacting solution is heated above its normal boiling point [23]. Here, similar to the co-precipitation method, alkaline KMnO4 solution and Ca2þ and Mn2þ was added into a 250 ml Teflon-lined autoclave. Before sealing the autoclave, 100 ml distilled water was added to this solution. The autoclave was kept at 180  C for 12 h and finally quenched naturally at room temperature. This admixture was neutralized by addition of distilled water, filtered, washed several times using water and ethanol, and calcined at 400  C for 4 h. Microwave method - Microwave sintering is used for synthesis of MMOs. In this method, the metal salts are subjected to microwave radiation at temperatures ranging from room temperature to several hundred degrees centigrade [24]. In this method, initially, the alkaline solution of KMnO4 was added into the solution containing Ca2þ and Mn2þ, and reacted under microwave irradiation (600 W, 5 s On/Off) for 2 min. The obtaining solution from above procedure was then mixed

Table 1 e Reaction condition for synthesis of CaMn3O6. Sample No. 1 2 3 4 5 6 7 8 9 10

Method

Ca ion precursors

Calcination temperature

Co-precipitation Reflux Microwave Hydrothermal Ultrasonic Ultrasonic Ultrasonic Ultrasonic Ultrasonic Ultrasonic

CaCl2 CaCl2 CaCl2 CaCl2 CaCl2 CaCl2 CaCl2 Ca(OAC)2 Ca(NO3)2 Ca(sal)2

400 400 400 400 400 700 1000 700 700 700

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with 25 ml of distilled water and again irradiated by microwave radiation (600 W, 5 s. On/Off) for 15 min. After 1-h irradiation, the admixture was primarily neutralized and finally calcined at 400  C for 4 h. Reflux method e Reflux method is an improved process for preparing crystalline MMOs in which an organometallic compound is reacted with a solution of metal ions in an alkaline environment [25]. For preparation of CaMn3O6 by reflux method, the adjusted alkaline KMnO4 and the solution of Ca2þ and Mn2þ were mixed in a 250 ml Round-bottom flask, where 100 ml of distilled water was added to this solution. The resulting admixture was refluxed for 24 h at boiling temperature. As the reaction was completed, the product was neutralized, filtered, washed, dried and finally calcined at 400  C for 4 h. Ultrasonic method e Sonochemistry is a method for preparation of nanosized MMOs. For preparation of CaMn3O6 nanoparticles, the solution of Ca2þ and Mn2þ was added into the alkaline solution of KMnO4 in a bath ice. The mixture was irradiated by the ultrasonic waves (a multi-wave ultrasonic generator - MPI Ultrasonics - welding, 1000 W, 20 kHz, Switzerland) for 20 min, where 100 ml of distilled water was added into the container and again irradiated for 30 min. The product was neutralized, filtered, washed, dried, and finally calcined at 400  C for 4 h. For measuring the output power, the temperature of the solutions was recorded versus time in the experiments. dT/dt was estimated from the plots of T (temperature) versus t (time) data. Then the power was calculated by mentioned formula [26]: power ¼

  dT cp M dt

(2)

where the cp is heat capacity of solvent (J kg1 K1) and M is mass of solvent (kg). The value of output power calculated about 16.2 W, in the distilled water (for the input power about 60 W).

Characterization Field emission scanning electron microscopy (FESEM - LEO 1455VP scanning electron microscope), X-ray diffractometer (XRD e Philips X'Pert Pro filtered with Cu Ka radiation operated at l ¼ 1.54
A), Fourier-transform infrared spectroscopy (FTIR Galaxy series FTIR-5000 spectrophotometer), transmittance electron microscope (TEM - Philips EM280 TEM), Diffuse reflectance UVevis spectroscopy (UV-DRS - Shimadzu UV/ 3101 PC in the range of 200e700 nm), and Brunauer-EmmettTeller (BET - nitrogen adsorption at 196  C using an automated gas adsorption analyzer Tristar 3000, Micromeritics) were used respectively for physico-chemical analysis of the CaMn3O6 nanoparticles. In addition, the atomic absorption spectroscopy (AAS) was performed using an analyst spectrometer system (Perkin Elmer 2380). Experimentally, appropriate amount of the sample was dissolved in HNO3 and H2O2 at room temperature and maintained for 24 h. The clear solution was switched into the AAS analyzer. The microprocessor-controlled electrochemistry meter AL20 from AQUALYTIC® meets the daily-to-day was demanded the robust and reliable systems for measuring temperature and

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Fig. 1 e FESEM micrographs of CaMn3O6 synthesized via (a) co-precipitation method, (b) reflux method, (c) microwave method, (d) hydrothermal method, and (e) ultrasound method after calcination at 400  C for 4 h.

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dissolved oxygen. This dissolved oxygen meter was used for measuring the evaluated O2 from water splitting reaction.

Catalytic water splitting The experiments of catalytic water oxidation were led on a round-bottom glass flask in a colorless three-neck contained 40 ml of water and Ce(NH4)2(NO3)6 and CaMn3O6. After bleeding with argon, one of the necks of the flask was locked with a septum. For measuring the evaluated oxygen, an oxygen sensor was inserted into the solution. Catalytic oxygen evolution from the water was conducted in the presence of (NH4)2Ce(NO3)6 by an Aloxi20 portable dissolved oxygen-meter which connected to a monitor with digital readout. The reactor of water-splitting reaction was maintained at 25 ± 0.5  C. Practically, the appliance readout was calibrated in air-saturated distilled, with continuous water stirring. In the absence of catalyst, the amount of oxygen in Ce(IV) solution was stable, while after bleeding of Ce(IV) with argon in the presence of CaMn3O6 nanocatalysts the oxygen evolution was recorded. From recorded data, the rate of oxygen evolution was attained from linear trends.

Results and discussion Structural analysis The surface morphology of samples was observed on their respective FE-SEM micrographs (Fig. 1). According to Fig. 1b, the surface morphology of reflux synthesized sample consists of the agglomerated particles with irregular clusters. However, the morphology and structure of other samples which formed by microwave (Fig. 1c), hydrothermal (Fig. 1d), and ultrasound (Fig. 1e) methods, confirmed the particle shape with nano size range. The optimized and small particle size with a dense texture can be observed in the sonochemically

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fabricated product. With respect to the surface morphology of all samples, the obtained sample which synthesized through ultrasonic method was selected for further physico-chemical investigations. The water splitting properties of all above morphologies will be discussed later. Fig. 2 shows FE-SEM micrographs of as-prepared products synthesized by ultrasonic method at different calcination temperatures (700  C and 1000  C). By increment the calcination temperature, the particle size was increased and the representative morphologies were modified to the tiny spherical structures. The best morphology according to the size and uniformity was achieved at 700  C. Fig. 3 illustrates FE-SEM images of as-prepared CaMn3O6 by ultrasonic methods using different Ca ion precursors involved Ca(OAC)2, Ca(NO3)2, and Ca(sal)2. The particle aggregation and irregular morphology can be observed in the presence of all type of calcium precursors. On the other hand, the size of nanomaterials was proliferated by changing the Calcium salts as precursors. Therefore, the salt of CaCl2 selected in the role of suitable precursor for preparation of CaMn3O6 nanostructures. The XRD patterns of CaMn3O6 nanoparticles prepared via co-precipitation method are shown in Fig. 4aef. As shown in Fig. 4aee, the provided samples through diverse chemical synthesis method and heat treated at 400  C have pure composition and well-matched with lattice structural phase of JCPDS 02-0630. Upon increasing the annealing temperature to 700  C, the obtained XRD pattern completely matched with JCPDS 31-0285 and confirmed formation of CaMn3O6 structures without any impurities (Fig. 4f). The average crystallite size of the system was calculated using Scherer equation Eq. (3): D ¼ Kl=bcosq

(3)

where b is the range of the detected diffraction line at its half intensity maximum, K is the alleged shape factor, which generally takes a value of about 0.9, and l is the wavelength of

Fig. 2 e FESEM micrographs of CaMn3O6 synthesized by ultrasound method after calcination for 4 h at the temperature of (a) 700  C (b) 1000  C.

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Fig. 3 e FESEM micrographs of CaMn3O6 synthesized by ultrasound method via different Ca ion precursors (b) Ca(OAC)2 (c) Ca(NO3)2 (d) Ca(sal)2.

the X-ray source that is used in XRD [27]. The average crystallite size of the optimized sample calculated around 25.3 nm. In order to confirm the obtaining crystallite size from Scherre equation, the crystallite size and lattice strain of the sample was also estimated via WilliamsoneHall technique [28] Eq. (4): bs cosq ¼ ðkl=dÞ þ 2ε sinq

(4)

where b is the full-width at half-maximum of the diffraction peak, l, q and ε are the X-ray wavelength, the diffraction angle and the lattice strain, respectively and d is the crystallite size. In WeH assumption, bs is Eq. 5 bs ¼ be  bi

(5)

where bi is the width at Si powder peaks in half-maximum that used to calibrate and be is the estimated width. So. Plotting the bs cosq against sinq gives a straight line with slope (ε) and intercept Kl/d. The crystallite size (d) is calculated from Kl/d and lattice strain is slop line (ε) [29]. The lattice strain and

grain size of the CaMn3O6 is obtained to be around 0.0071 and 28 nm, respectively. The elemental analysis of CaMn3O6 nanostructures confirm the purity of the samples, as proposed initially from XRD analysis (Fig. 5). The EDS reveals the chemical composition of products which composed of only Ca-, Mn- and Owithout any impurity peaks. The ratio of Ca:Mn and Ca:O are obtained to be around 1:3 and 1:6, respectively. As the co-precipitation treatment is used to yield a pure MMOs, the phase formation based metal-oxygen (M-O) and metal-oxygen-metal (M-O-M) bonds are critical to clarify. Fig. 6 shows the FTIR spectrum of the CaMn3O6 nanoparticles after calcining at 400  C by different precursors of Ca ions in ultrasonic method. In this spectrum, seven bands at 3451, 1643, 1419, 834, 627, 518, and 436 cm1 can be observed. A wide absorption at 3451 cm1 is assigned to the stretching vibration of eOH, and a peak at 1643 cm1 is allocated to the bending vibration mode of adsorbed water. A band at 1419 cm1 is assigned to the stretching mode of MneO. Correspondingly, two small bands at

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Fig. 4 e XRD patterns of as-prepared product via (a) coprecipitation method, (b) reflux method, (c) microwave method, (d) hydrothermal method, and (e) ultrasound method after calcination at 400  C for 4 h and (f) ultrasound method after calcination at 700  C for 4 h.

Fig. 5 e EDS spectrum of CaMn3O6 synthesized via coprecipitation method at 700  C for 4.

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Fig. 6 e FTIR spectrum of CaMn3O6 synthesized via coprecipitation method at 700  C for 4 (a) CaCl2 (b) Ca(OAC)2 (c) Ca(NO3)2 (d) Ca(sal)2. 834, and 518 cm1 arise from the stretching vibration frequencies of MneO and MneOeMn [30], whereas two peaks at around 627 and 436 cm1 are assigned to the stretching vibration frequency of Mnþ3- O6, and Mn4þ- O6, respectively [31]. The DRS spectra of CaMn3O6 nanoparticles is shown in Fig. 7. Due to the presence of defects in MMOs, and also Mn ion, with different crystal field and coordination, a series of individual valence states in the crystal lattice of the CaMn3O6 is proposed [31]. In this structure, the d-electrons of Mn ion can split into two energy levels; eg and t2g. The optical band gap of CaMn3O6 can be estimated from the optical density (ahn)2 versus hn plot. The estimated value of the direct band gap is ~5.3 eV, in a good agreement with the literature reported elsewhere [32]. Composition, crystal structure, particle size, and surface area are four important parameters which govern the activity of the catalysts [31]. The specific surface area of the CaMn3O6 nanostructures, obtained from BET analysis, was observed to be around 15.16 g/m2. All BET-BJH factors are tabulated in Table 2. Fig. 8aec prove the BJH and nitrogen adsorption/desorption profiles of the CaMn3O6 nanostructures. This diagram supports the IUPAC H3 classification. The H3 mode hysteresis is attributed to the solid powders, which contain agglomerate or

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Fig. 7 e DRS spectrum of CaMn3O6, and Tauc plot of the sample synthesized via co-precipitation method at 700  C for 4 h. aggregate nanostructures with the channel shape holes and pores, having an irregular shape. The total pore volume and the mean pore size of the sample are around 0.051 ± 0.0003 cm3 g1 and 13.532 nm, respectively. The mean pore radii (rp) for the sample is obtained to be around 3.750 nm. It is known that in the dispersed solids with rp between 1 and 25 nm (1rp 25 nm), the nanoparticles are composed of aggregates [32]. This is in good agreement with the FE-SEM results. Dense, nanosized, and crystalline particles in the TEM micrographs of CaMn3O6 nanoparticles can be observed in Fig. 9. In these four magnifications, a strong contrast indicates the high density and crystallinity of the particles. The longitudinal lengths of the particles are in the range of 14e26 nm which agree with FE-SEM and BET observations.

Water splitting and O2 evolution reaction Hydrogen generation and O2 evolution are two major keywords which recognized as a solution for future energy. As mentioned before, photocatalytic splitting is a process in which the energy of light trapped in first step and finally converted into solar fuels. In a typical water splitting system, turnover number (TON) and frequency (TOF) are two important and considerable factors.

Table 2 e BET-BJH parameters of CaMn3O6 nanoparticles. Plot data BET plot Vm as BET C Total pore volume (p/p0 ¼ 0.990) Mean pore diameter BJH plot Vp rp,peak(Vol) ap

Desorption branch 3.4826 15.158 98.738 0.051278

(cm3.g1) [STP] (m2.g1)

13.532

(nm)

0.051354 3.75 17.779

(cm3 g1) (nm) (m2 g1)

(cm3.g1)

(NH4)2Ce(NO3)6 is a powerful and non-oxo transfer oxidant, which used widely in oxidation pathway of water to oxygen [33,34]. This complex can be catalyzed by manganese compounds. Fig. 10 shows the O2 evolution profile of oxidation pathway by CaMn3O6 nanocatalysts synthesized via ultrasonic reaction. The evaluation of catalytic activity for this sample was tracked using various concentrations of (NH4)2Ce(NO3)6. The TOF and TON profiles were quantified the specific activity of a host (catalytic center) at the center per unit time. While TON is used for estimation of the permanency of a catalyst system, the TOF is measured the real kinetics of a reaction [35]. The maximum amount of O2 was evolved in the sample containing 4 gr oxidant. Similarly, the highest TOF and TON are recorded in the sample with 4 gr Ce(IV). It can be deduced that the amount of evaluated O2 can be enhanced in the presence of Ce(IV) in aqueous solution. Upon increasing the amount of Ce(IV) in the solution, the accessibility of oxidant and catalyst are increased (Table 3). In order to study the effect of morphology and particle longitude on the catalytic behavior of the CaMn3O6, various samples which synthesized by ultrasonic reaction (Section Structural analysis) were selected by considering their respective diagrams of oxidation reaction (Fig. 11). It was observed that the size of the particles and homogeneity of the texture directly affect the catalytic activity of the particles. Among reflux, microwave, ultrasonic, and hydrothermal methods, superior TOF and TON were obtained for the sample which synthesized via ultrasonic method. From its respective FE-SEM micrograph, the ultrasound CaMn3O6 have a very small particle longitude with a uniform texture (Table 3).

Discussion water splitting results The structure of manganese-calcium oxide on photosystem (II) The hydrogen generation from decomposition and oxidation of water can be recognized as a suitable solution for energy crisis in future. Apply solar energy to lay out artificial photosynthesis is one of the passible strategy that means to trap the light energy and produce solar fuels (most commonly H2) using the electrolytic reactions [36,37]. The anodic reaction for

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atmosphere of earth. Photosystem II contributes to photosynthesis process that is using of the energy of sunlight to produce charge separation and lead to reduce of plastoquinone to plastoquinol and oxidation of water to oxygen [41e43]. The reducing energy for this reaction completed by another photonic energy that absorbed in photosystem (I) which is produced by NADPH and ATP. These structures applied to assimilate the CO2 in the Calvin-Benson cycle of the photosynthesis which named “dark reaction” [44]. By absorbing the energy of light or sunlight in photosystem II in a four electron reaction the water oxidize and formed dioxygen: 2H2 O / O2 þ 4Hþ þ 4e

Fig. 8 e (a) N2 adsorption/desorption isotherm, (b) BET plot, and (c) BJH diagram of CaMn3O6 nanoparticles synthesized via ultrasonic method.

generation of hydrogen by oxidation of water is strongly ratelimiting, as a result in considerable electrode over-voltage (~1 V) that work at current densities required practical operations and conversion efficiencies was directly affected by these impacts. High over voltage was cause to form other chemical materials at anode, and it's not acceptable for the generation of hydrogen in large scale. So, finding and designing a suitable super catalyst for the reaction of anodic oxidation and generation of hydrogen is the most important challenge in hydrogen economy [38] In nature, the water oxidation process is catalyzed by a complex (CaMn4O5) located in photosystem (II) in a protein rich environmental which controlled the movement of protons, reaction coordinates, and the access of water [39,40]. In photosystem II, CaMn4O5 was catalyzed oxidation of water and it is known as a responsibility for the presence of oxygen molecules on

(6)

In the plants, cyanobacteria and algae the oxidizing of water was done by a super catalyst which made of from four Mn ions and one Ca ion and two Cl ion that carried out this reaction. The manganese ion has a unique role in oxidizing of the water in photosystem II [45]. According to the studying of research group about the structure of water oxidation complex, it was shown that there are different models of CaMn4 structure. In this cluster four manganese and one calcium ions were bridged by five oxygen. In this structure, four molecules of water existed that two of them act as a substrate for the oxidation of water. The structure of CaMn4O5 is a distorted chair by an asymmetric cubane serving. In this chair the seat base made of from Mn4 and the back of the chair made of from O4 [46]. The distance of inter metal MneMn in this cluster is in good agreement with the symmetrical trigonal prism of CaMn3 and researchers were postulated to four bridge of oxygen linked to this structure and shaped the tetrahedral array of CaMn3 and other Mn located in the external of cube and the other oxygen bridged to this Mn [47]. According to these researches in this work, we synthesized CaMn3O6 as a catalyst for water oxidation to study the effect of CaMn3 on water oxidation. CaMn3O6 is built up with double chains of edge-sharing MnO6 octahedral. The six sided tunnels are created by cornerlinked double chains for the cation of Ca2þ. Compared to the other phases of the CaxMnyOz structure such as CaMnO3, Ca2Mn3O8, CaMn2O4 has a great motivation for investigation the catalytic properties because of the structural diversity of perovskite, layer and post spinel phases in the present CaeMne O series. To compare the post-spinel phase of CaMn2O4, one third of the Ca position is vacant in the tunnels of structure of CaMn3O6, and could act as a proper position for catalytic activity [48]. The presence of the double chains of MnO6-octahedron and the stability of these tunnel structures made increase the electrical and magnetic properties of these materials [49].

Oxygen evolution mechanism by calcium manganese oxides nano-catalyst Mechanism of water oxidation by water oxidizing complex (WOC) is one of the great unknown problems in biomimetic and bioinorganic chemistry. Fundamental platform function of the WOC was developed by Kok and Joliot and was known as Kok cycle [50]. In this cycle, the transition from S0 to S4 was done by light irradiation but the transition from S4 to So was independent to light [51]. There are existed some proposals explained the oxygen evolution with the WOC in photosystem II. Some of these

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Fig. 9 e TEM images of as-prepared CaMn3O6 by ultrasonic method.

models were based on nucleophilic attack. According to research studies, Mn-bound species are extremely electrondeficient for instance Mn(V)]O. The proposed mechanism for water oxidation is that the terminal Mn(V)]O withstand a nucleophilic attack by Ca2þ bound to hydroxide and form Mn-bound hydroperoxide. So, the Ca2þ ion was played the role of week Lewis acid. Water molecules bounded to Calcium ion were reacted with Mn(V)]O and were formed O]O [52,53]. Four distinct mechanisms proposed water oxidation by the same manganese oxide structures described in below: A: Thermal reduction of catalyst by water

The oxidation number of manganese is 3 þ and 4þ. It's practical that oxidation of water and reduction of manganese produces thermally. This mechanism was not depended to the absorption of light. The rate of this reaction is very slow and this reaction was done in dark condition 2H2 O 4 O2 þ 4Hþ þ 4e E0 ¼  1:23 V

(7)

Mn2 O3 þ 6Hþ þ 2e42Mn2þ þ 3H2 Ok E0 ¼ þ 1:485 V

(8)

MnO2 þ 4Hþ þ 2e4Mn2þ þ 2H2 O E0 ¼ þ 1:23 V

(9)

Additionally, it is clear that presence of Ce(IV) in this system being to photolysis. The presence of Ce(IV) has a

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 3 7 3 e2 6 3 8 6

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significant and positive effect on the rate of evolution of oxygen. According to context of this mechanism, the reaction of the manganese and Ce(IV) was listed as below reaction: Mn2þor3þ þ Ce4þ /MnO2 ðorMn2 O3 Þ þ Ce3þ

(10)

The reduction potential of Ce4þ/3þ is 1.4 V. In this mechanism, the speed of oxygen evolution was increased in presence of this oxidant [54,55]. B: Electron-hole chemistry

Fig. 10 e O2 evolution profile of CaMn3O6 nanocatalysts synthesized via ultrasonic method after calcination at 700  C in the presence of various concentrations of Ce(IV).

Table 3 e Water splitting results of CaMn3O6 nanoparticles prepared by ultrasonic method. Sample No 6 6 6 6 7 8 9 10

Parameter

Amount of Ce(IV) Amount of Ce(IV) Amount of Ce(IV) Amount of Ce(IV) Temperature Ca ion precursors Ca ion precursors Ca ion precursors

TOF (mmol O2. mol Mn1 s1)

TON (mmol O2. mol Mn1)

0.0023 0.0043 0.0097 0.037 0.027 0.024 0.022 0.016

0.35 0.65 1.64 5.55 4.05 3.60 3.30 2.40

Mechanism B relied on electron-hole chemistry for the evolution of oxygen. Manganese oxide is a semiconductor and it was excited by visible light and an electron donate to the acceptors presented in the system. One of the Possible electron acceptors is Ce(IV). If this electron donates to water, the results of this reaction are evolution of hydrogen. Therefore, in the lack of hydrogen evolution illustrates that the electron donation to water does not occur. Transition metal oxides were known as the poor water-splitting catalysts due to the filled d-orbitals served as centers for recombination of electron-hole. This hole quenched via water oxidation was listed in the below reaction [56,57]: Mn3þor4þ þ hy/Mnð3þor4þÞ*

(11)

Mnð3þor4þÞ* þ acceptor/Mn4þor5þ þ hþ

(12)

2H2 O þ 2hþ /O2 þ 2Hþ

(13)

C: Direct oxygen evolution from catalyst It was proposed that the bulk oxygen in the form of O2 immigrates to surface of manganese oxide catalysts under illumination of visible light. Visible light was activated the release of surface oxygen by photolyzing of MneO bonds and release the oxygen [58]. O2 ðsÞ þ hy þ e /O ðsÞ þ hn þ e /O2 ðsurfaceÞ

(14)

O2 ðsurfaceÞ þ hy/O2 ðgÞ

(15)

D: Catalysis of Ce(IV)/Ce(III) water oxidation by manganese oxide

Fig. 11 e O2 evolution profile of CaMn3O6 nanocatalysts synthesized in different conditions.

Nanostructures of manganese oxide clusters are efficient oxygen evolving catalysts using a photosensitizer and an electron acceptor. Manganese oxide catalyst was acted the catalyst role for the water oxidation by Ce(IV)/Ce(III) and catalyzed oxidation of the water by Ce(IV). This mechanism is affected by the presence of Ce(IV) and illumination [59]. Nanosized manganese oxides, especially in the form of MMOs, are potential for water oxidation reaction. Considering the other catalysts containing Mn, the nanosized MMOs with

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uniform texture are more efficient for water oxidation reaction. Several perspectives were considered to ascribe the catalytic O2 evolution via water oxidizing complex in photosystem II model [60]. This model is assumed based on a nucleophilic reaction mechanism. For example, in the MMOs, the terminal dMn(V)]Od is potential for nucleophilic attack by calcium ion (Ca2þ.) This is bounded to the hydroxide, therefore, Mn-bound hydroperoxide can be formed. The calcium ion plays as a week Lewis acid [35]. The bound H2O on the surface of Ca2þ were reacted with dMn(V)]Od, as a result, the O]O molecules are evolved. The presence of oxidation agents like cerium (IV) ions (Ce4þ) can affect catalytic O2 evolution. Such a mechanism can be proposed according to the following assumption (Eq. (15)e(19)): Ce4þ þ H2 O/½CeðOH2 Þ4þ

(16)

Ca2þ þ H2 O / ½CaðOH2 Þ2þ

(17)

Mn5þ ¼ O þ ½CaðOH2 Þ2þ / OdMn4þ dCadO þ 2H

(18)

OdMn4þ dCadO þ ½CaðOH2 Þ4þ / Ce3þ dOdOdMn3þ þ CaO þ 2Hþ Ce3þ dOdOdMn3þ þ H2 O/ Ce3þ þ Mn2þ ðOH2 Þ þ O2

(19) (20)

In these mechanisms, the oxo-hydroxocerium (IV) radical coupling is formed, in which the cerium (IV) acts as an oxidant. It is an efficient and most possible pathway for reaction between the catalyst and cerium (IV) [17,35].

Conclusion In summary, a catalytic system presented in this work are based on MMOs containing Mn and Ca ions. Experimentally, the CaMn3O6 nanostructures are successfully synthesized via wet-chemistry methods. As far as we know, under homogenous conditions, the Mn containing MMOs are the most active photocatalysts which exhibited the highest values of TON and TOF. Based on this approach, the nanosized CaMn3O6 has been proposed as an active texture for O2 evolution reaction. Interestingly, the catalytic performance of this catalyst has allowed us to establish that the morphology and the size of the catalyst are two structural key aspects for a potential water splitting. The samples with smallest particle longitudes (those which synthesized via co-precipitation and ultrasonic techniques) are highly motivated in the conceptual design of water splitting materials. The O2 evolution activities of CaMn3O6 nanocatalysts in the presence of a supported oxidant ((NH4)2Ce(NO3)6) indicate that upon increasing the concentration of oxidant to 4 gr, the TOF and TON are enhanced to 0.0274 mmol O2. mol Mn1. s1 and 4.1110 mmol O2. mol Mn1, respectively. Mn-based nanocatalysts described herein can fill up the gap in the design of solid-state water splitting systems, however, more investigations are required in order to enable MMOs for future energy systems.

Acknowledgment Authors are grateful to the council of Iran National Science Foundation (97014327) and University of Kashan for supporting this work by Grant No (159271/1390).

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