Grafting of Ag nanoparticles on SrMnO3: Effect of capping agent and alkaline agent on the morphology of SrMnO3 nanostructures for enhancing photocatalytic performance

INTRODUCTION

Newly, water cleaning is chosen as a serious challenge, therefore is known as one of the most significant fields in scientific research and organic dyes are the major pollutant types of wastewater. Up to now, numerous attempts has been dedicated to decline or demolition of pollutants in the wastewater. Of all the offers, photocatalyst way is known as one of the best and most promising routes for deletion organic contaminants from water [1-5]. In these processes, organic molecules decompose by interacting with a photo-catalyst material and UV or visible light and finally CO2 and H2O are achieved [6-9].

Different compounds of manganese used in various fields such as catalysis, batteries, photocatalysis, magnetite, and electrochemistry [10-12]. In recent years, many of the research groups studied the mixed-valent manganite AMnO3 (A= Ca, Sr, Ba). The interesting properties of manganese oxides originated from the promise of technological applications [13]. Most of the studies concentrated on the structures by A1-xRxMnO3 formula. However, it found that a low number of works has been studied the AMnO3 [14-16]. The SrMnO3 is extensively used as antiferromagnetic insulator with a crystal structure of G-type [17, 18]. Due to the special skeletal structure which was consisted of a three dimensional network of MnO6 octahedra, the SrMnO3sample has some properties such as oxygen nonstoichiometry, electronic properties, thermochromism, etc [19].

There are many methods for the synthesis of nanoparticles, including: sol-gel, hydrothermal, solvothermal method, reverse micelle methods, thermal decomposition, solid-state and sonochemical approach.

SrMnO3 powders have been synthesized with many methods such as high temperature solid-state reaction, coprecipitation [20], sol-gel, etc [21-24]. High temperature solid-state reactions need a very high temperature. Furthermore, obtained products with this technique are agglomerated in a big grain size. Coprecipitation and sol-gel processes can prepare nanopowders in a lower temperature. Among the above methods for proportion particles in nanoscale, the co-precipitation approach is well-known as a high efficiency in the production of nanoparticles, simple process with high usability, cost-effective and production of high percent purity nanoparticles. Also this way is a very promising technique for the preparation of fine particles for mixed oxides, allowing controlled size and shape with commonly good combinatorial uniformly. In this work, we have reported the prepared of SrMnO3 nanoparticles by co-precipitation process and then characterization by diverse techniques such as UV–Vis diffuse reflectance spectroscopy (DRS), energy dispersive X-ray microanalysis (EDX), Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). Then we examined the effects of various capping agents, alkaline agents and reaction temperature to achieve optimum conditions. Now the ability of SrMnO3 as photocatalyst for the destruction of the organic dye including rhodamine B, methylene blue, murexide and methyl orange (as the model water pollutants) were studied. The effect of several factors such as type of pollutant, particle size and shape of SrMnO3 nanostructures, pH and concentration of pollutant on photocatalytic behavior of products was measured.

MATERIALS AND METHODS

Materials and Characterizations

Materials applied in this were Strontium nitrate (Sr(NO3)2, Merck, 99.9%), Manganese nitrate (Mn(NO3)2, Merck, 99.9%), AgNO3, dimethylglyoxime (DMG), 2-hydroxy-1-naphthaldehyde, triethylenepantamine (abbreviated TEPA) and NH3. All chemicals were used without further purification. Also de-ionized water was used as solvent. The XRD of products was recorded by Philips, X-ray diffractometer using Ni-filtered Cu Kα radiation. The EDX analysis with 20 kV accelerated voltage was done. FT Infrared (FT-IR) spectra were obtained as potassium bromide pellets in the range of 400–4000 cm-1 with a Nicolet-Impact 400D spectrophotometer. SEM images were taken using an LEO instrument model 1455VP. Prior to taking images, the samples were coated by a very thin layer of Pt (using a BAL-TEC SCD 005 sputter coater) to make the sample surface conductor, to prevent charge accumulation, and to obtain a better contrast. Transmission electron microscope (TEM) images of the as-obtained SrMnO3 nanostructures were taken on a JEM-2100 with an accelerating voltage of 200 kV. The UV–Vis diffuse reflectance spectrum of the as-produced SrMnO3 nanostructures was obtained on a UV–Vis spectrophotometer (Shimadzu, UV-2550, Japan). Thermogravimetric analysis (TGA) was carried out using an instrument (Shimadzu TGA-50H) with a heating rate of 10 °C min-1 under nitrogen atmosphere.

Synthesis of SrMnO3

The SrMnO3 sample was synthesized using strontium nitrate, Manganese nitrate, TEPA and de-ionized water as solvents. In a typical method, 1 mmol of strontium nitrate and 1 mmol manganese nitrate individually dissolved in 20 ml of de-ionized water and then two solutions are mixed on magnetic stirrer at room temperature to achieve a homogeneous solution. Then 2 mmol of DMG, as a new capping agent, was added to the above solution and after it was stirred for 30 minutes. After 30 minutes stirring, triethylenepantamine as alkaline agent and capping agent was added dropwise into the mixed solution of Mn and Sr (pH= 10) and the temperature of the solution was maintained at about 70 °C. The formed precipitate was separated through centrifugation, washed by distilled water and ethanol, air-dried and at last calcined at 1000 °C for 5h. In Table 1 the production conditions of SrMnO3 structures contain kind of capping agent and alkaline agent that effect on the shape, purity and grain size has been illustrated. For assessment the role of triethylenepantamine and DMG as a new precipitant and capping agent respectively, two samples were synthesized by NH3 and 2-hydroxy-1-naphthaldehyde.

Synthesis of SrMnO3/Ag nanocomposite

SrMnO3/Ag nanoparticles were synthesized by photodeposition route. A suitable amount of AgNo3 was dissolved in 25 ml deionized water and then 0.05 g SrMnO3 was added to the above solution under magnetic stirring for 20 min. Next, 0.5 ml ethanol Merck added to solution. Deoxygenation was also conducted simultaneously. Then the solution was transferred to quartz tube and stirred under UV irradiation for 3 hours. The formed product was separated and washed with deionized water and ethanol three times and dried at 70 °C Fig. 1.

Photocatalytic process

The photocatalytic activities of as-synthesized SrMnO3 and SrMnO3/Ag samples were done by monitoring the demolition of methylene blue, murexide, methyl orange and rhodamine B in aqueous solution, under irradiation with UV light. The maximum absorbance wavelength for rhodamine B, methylene blue, methyl orange and murexide are 543, 668, 510 and 506 nm respectively. The reaction mixture comprising of the 50 ml of the contaminate solutions and 50 mg of SrMnO3 or SrMnO3/Ag in the glass reactor was used to determine the photocatalytic process. The mixture of contaminate and catalyst was placed in photoreactor under UV light and stirred for 20 min at dark. The photocatalytic tests were accomplished in a glass vessel with a diameter of 10 cm. A 400W Xe lamp was used as light source. After 10 min, 5 mL of the suspension was sampled and centrifuged. Then, the absorbance spectra of the dyes solution were recorded with a UV-Vis spectrophotometer, and the rate of degradation was observed in terms of change in intensity at λmax of the dyes. The organic contaminant photodegradation efficiency has been determined as follow:

Degradation (%) = (1)

where C and C0 are the absorbance quantity of organic contaminant solution before and after decomposition by a UV–Vis spectrometer, respectively [25-27].

RESULTS AND DISCUSSION

Thermogravimetry (TG) technique was used to examine of the thermal stability of the as-synthesized nanostructure. Fig. 2 demonstrates the TG curve of the as-prepared powders (sample No. 1 before calcination steps). In this figure, there are three mass loss stages. The happened endothermic weight loss in the temperature range of 45–250 °C (indicating 4.98 % weight loss) related to the dehydration of the water remaining in the as-synthesized nanostructures. One happened endothermic stage at the temperature range of 250–1000 °C (showing 14.52 % weight loss) may be corresponded to removal of the nitrates and demonstrate the preparation of SrMnO3 nanostructures.

XRD analysis, which is the useful technique for assessment of crystalline structure, was employed to investigate the synthesized SrMnO3 nanostructures. Fig. 3 show the XRD pattern of the SrMnO3 nanostructure prepared with Mn(NO3)2 and Sr(NO3)2 precursors. The XRD results for sample 5 before and after calcination are shown in Fig. 3. The presence of (1 0 4), (1 1 0), (1 0 5), (2 0 2), (2 0 3), (1 0 7), (2 1 3), (3 0 0), (1 1 8), (1 0 9) and (2 2 0) peaks in the XRD pattern of SrMnO3 (Fig. 3a) are in accordance with hexagonal structure and JCPDS standard cards no. 24-1221. From the peak of the reflection (1 1 0) of SrMnO3, the particle sizes of the samples was estimated using Sherrer’s equation [28-30], Dc=Kλ/βcosθ, Where K is the so-called shape factor, which takes about 0.9, β is the breadth of the observed diffraction line at its half-intensity maximum and λ is the wavelength of X-ray source used in XRD. The average size of the as-obtained nanoparticles was estimated to be 48 nm. By applying DMG and TEPA as capping agent and alkaline agent, pure SrMnO3 nanostructures have been synthesized (Fig. 3a). As can be seen the crystalline structure and purity of compound is desirable. Fig. 3b demonstrates XRD pattern of sample 4 before calcination, in this case the crystalline structure of SrMnO3 is not desirable.

Then to show Ag in structure, elemental analysis (EDS) analysis was employed. EDX analysis of SrMnO3/Ag nanostructure is showed in Fig. 4. The lines of Sr, Mn, Ag and O are clearly observed and there were no other peaks for impurities.

FT-IR is a rampant analytical that applied to identify organic materials in middle IR region and in some cases inorganic material in far IR region. Fig. 5a shows FT-IR result for synthesized SrMnO3 nanoparticles (sample 5) before calcination. The broad band at 1620 cm–1 was assigned to the oxime group in DMG structure. The peak nearly 1384 cm–1 is due to stretching vibration of the amine group. The presence of broad band at 3430 cm -1 is accordance to the stretching vibrations of surface hydroxyl (–OH) groups. The band at around 841 cm-1 is dedicated to the C-H and CH2 vibrations of TETA and DMG. In Fig. 5b two peaks at 482 and 603 cm-1 refers to the stretching vibration of the metal-oxygen bond. The presence of these bands indicates that Sr and Mn were situated in an oxygen environment in the hexagonal structure. Also the broad band at 3438 cm–1 was assigned to the stretching vibrations of surface hydroxyl (–OH) groups.

The morphologies of nanomaterial were studied by scanning electron microscopy. FESEM technique was used to evaluate the influence of factors such as the kind of capping agent and alkaline agent on the grain size and morphology of the SrMnO3 nanoparticle. To investigate the role of the kind of alkaline agent on the size and morphology of SrMnO3, NH3 and TEPA were utilized (sample Nos. 2, 3). As can be seen in Fig. 6c and d, the synthesized nanoparticles using TEPA as alkaline agent, are more distinctive and less agglomerated in compared with those that prepared using NH3 (Fig. 6a and b). As a result TEPA was chosen as optimum alkaline agent. The TEPA acts as both alkaline agent and capping agent through preventing the aggregation of the prepared nanoparticles. Now by using of TEPA and precursors of Sr and Mn effects of capping agents were investigated. Fig. 7 demonstrates FESEM images of sample Nos. 4 and 5 prepared in the presence of the 2-hydroxy-1-naphthaldehyde (Fig. 7a and b) and DMG (Fig. 7c and d). Among these the prepared samples, sample No. 4 synthesized by using DMG in presence of TEPA, show homogeneous sphere-like nanoparticles with small particle size. It seems that among were employed capping agent and alkaline agent, DMG and TEPA has the highest steric barrier effect. Furthermore, FESEM images reveals that the increment in the steric barrier effect causes to the grain size of samples becomes small. It seems that the nucleation to be happened rather than the particle growth through increasing the steric barrier effect. Now on the surface of the synthesized nanostructure using TEPA and DMG, which known as the optimum sample, silver nanoparticle was deposited by photodeposition method. Fig. 8a and b depict morphology of SrMnO3/Ag nanocomposite. As can be seen surface and size of SrMnO3/Ag nanostructure (sample 6) is rough and elder in comparison with sample 5 nanostructure, which is evidence for coting of Ag particles on the surface.

Figs. 9a and b demonstrate TEM images of SrMnO3/Ag nanostructures prepared via Strontium nitrate and Manganese nitrate and Ag(NO3) precursors in presence of TEPA and DMG as alkaline agent and stabilization agent respectively. The nanostructures with narrow size distribution are clearly seen in TEM images, which are consistent with the above-mentioned SEM analysis (7a, b).

In order to determine the band gap of the SrMnO3 and the SrMnO3/Ag nanostructures, UV–Vis diffuse reflectance analysis was taken. Fig. 10a and b reveals the UV–Vis absorption spectrum of sample No.5 (prepared using TEPA in presence of DMG as capping agent) and 6 (SrMnO3/Ag). The peaks nearly 240, 255, 262, 300, 325, 352, 375 and 392 nm (Fig. 10a) can be seen in the DR-UV-Vis spectrum of sample 5. The band gap can be determined based on the absorption spectrum using Tauc’s equation [31, 32]. The band gap value of the nanostructures was obtained through extrapolating the linear section of the plot of (αhν)2 against hν to the energy axis (Fig. 10c). The Eg value of the as-prepared SrMnO3 nanostructures estimated to be 3.22 eV. Fig 10b demonstrates absorption bands at 220, 247, 256, 275, 292, 303, 322, 340,398 and 403 nm for SrMnO3/Ag nanostracture. The Eg of the SrMnO3/Ag nanostructure (sample no. 6) estimated to be around 3.1 eV (Fig. 10d). From the calculated Eg value, as-prepared SrMnO3/Ag and SrMnO3 samples may be employed as the photocatalyst under UV illustration.

The role of various stabilization agents and alkaline agent on the photocatalytic efficiency of SrMnO3 has been investigated through monitoring the destruction of various dyes pollutant under ultraviolet illumination. The influence of various factors including kind of pollutant, grain size and morphology of SrMnO3 nanoparticles, pH and amount of photocatalyst on photocatalytic activity of products were examined. Reaction model of the SrMnO3/Ag nanostructure for photocatalytic activity is proposed as shown in Fig. 11.

Also the offered mechanism of the decomposition of dyes can be summarized as:

SrMnO3/Ag + hν → SrMnO3/Ag * + e- + h+

h+ + H2O → OH· + H+

2e− + O2 + 2H+ → O2-.

OH. + O2-. + Pollutants → Destruction products

During the photocatalytic process, the generated electron and holes may migrate to the catalyst surface where they participate in redox reactions with adsorbed species. The h+ may interact with H2O to product the hydroxyl radical (OH·). Also e – may interact with O2 to produce the oxygen radical (O2-.). It has been suggested that the hydroxyl radicals (OH·) and oxygen radical (O2-.) are the essential oxidizing species in the photocatalytic perocess. For blank sample no dye degradation was observed after 70 min without using UV light irradiation or photocatalyst. In the base of this observation, the contribution of self-degradation is insignificant.

The photocatalytic activity of samples 4, 5 and 6 was evaluated for demolition of rhodamine B, the results are shown in Fig. 12. As it can be seen photocatalyst performance of sample 6 (SrMnO3/Ag nanostructure) is much higher than that of samples 4 and 5. From the photocatalytic calculations, the rhodamine B contaminate destruction were about 54% by sample 4, about 63% sample 5 and about 70% by sample 6 after 70 min UV irradiation. By comparing the SEM images of as-prepared nanostructures (Figs. 7a-d) it can be observed that the use of TEPA and DMG (sample 5) is satisfactory for the production of uniform spherical SrMnO3 nanostructure. Because whatever the particle size is smaller the ratio of surface area to volume increases, therefore the catalytic performance is better. The photocatalytic activity of sample 6 (SrMnO3/Ag) is better than other samples, because of two reasons; first, Ag particles were selectively deposited on the electron trapping sites in photodeposition process, so the resulting Ag particles will serve as new electron sumps to entrap electron and extract of them to demolition of dye. Second, it is generally admitted that the recombination of electron (e-) and hole (h+) has a significant impact on the photocatalytic performance. By decreasing the recombination of electron (e-) and hole (h+), the photocatalysis activity is increased. When the band gap decreases, the chance of recombination of electron (e-) and hole (h+) can be decreased, whereupon photocatalysis activity increases. In the case of nanostructured SrMnO3/Ag with lower energy gap quantity then SrMnO3, the electron and hole recombination is reduced and therefore the better photocatalytic performance is observed [33]. In the next step, the photocatalytic performance of SrMnO3/Ag was investigated for decomposition of MO, murexide and MB, and the results were shown in Fig. 13. The catalysis efficiency of methyl orange, murexide and methylene blue are 35%, 45% and 58% respectively. It is evident that the photocatalytic destruction of anionic contaminants with a negative charge is weaker than cationic contaminants with a positive charge. The greatest amount of photocatalytic decomposition is for rhodamine B. These results reveal that in photocatalytic process, the cationic pollutant adsorption on the SrMnO3/Ag is better than of adsorption of anionic contaminant on it. Forasmuch as the nanostructured SrMnO3/Ag has oxygen atoms with a great electron density on its surface, it seems that SrMnO3/Ag with negative charge can adsorb the cationic pollutant with positive charge. Fig. 14 reveals the effect of various amount of photocatalyst on the photocatalytic performance of SrMnO3/Ag nanostructure. It can be seen that dye removal efficiency increased with the increase of photocatalyst concentration. The results illustrate that the percentage decomposition of rhodamine B under UV light increased from 70% to 78% with changing amount of photocatalyst from 50 to 70 mg in 50 ml of the contaminate solutions. The pH of the solution is known as effective parameter in the photocatalytic reactions. For investigate of the pH role on the destruction rate, two values of pH were tested. Destruction of rhodamine B was measured under acidic (pH=4) and neutral conditions (pH= 7) (Fig. 15). From photocatalytic calculations, the rhodamine B pollutant degradation was about 55% at pH= 5 and 70% at pH= 7 after 70 min UV illustration. By reducing pH, absorption of rhodamine B on the surface of the nanocatalyst is reduced. As shown in Fig. 16, in acidic and basic conditions the surface of nanostructures is covered by more positive and negative charge respectively. In acidic condition cationic molecules can be more desorbed on the surface. Subsequently, it can be seen that maximum demolition for rhodamine B was obtained in the neutral condition.

CONCLUSION

In this work, SrMnO3 nanostructures were prepared by co-precipitation method. The effect capping agent, alkaline agent and calcination temperature were investigated on size and morphology of product. Results show that the best product has been achieved in presence of TEPA and DMG. In the next step, silver particle deposited on this pure nanoparticle. Results show that preparation conditions have been influenced on size and morphology of nanostructures and have the most important effect on the photocatalytic performance. The influences of various factors including kind of pollutant, grain size of SrMnO3 nanostructures, amount of SrMnO3/Ag and pH on photocatalytic activity of products were evaluated. It was observed that by using of SrMnO3/Ag nanostructure instead SrMnO3 nanoparticles the photocatalytic performance increased. Also results suggest that SrMnO3/Ag nanoparticles are interesting and desirable candidate for photocatalytic applications under UV light.

ACKNOWLEDGEMENTS

The authors are grateful to Islamic Azad University (IAU) for providing financial support to undertake this work.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.