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Synthesis, characterization and photocatalytic studies of MCM-41 mesoporous silica core-shells doped with selenium oxide and lanthanum ions
Microporous and Mesoporous Materials 292 (2020) 109714
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Synthesis, characterization and photocatalytic studies of MCM-41 mesoporous silica core-shells doped with selenium oxide and lanthanum ions
T
Samaneh Esmaeilia,∗, Mohammad Ali Zanjanchia,∗∗, Hossein Golmojdeha, Shahab Shariatib a b
Faculty of Science, Department of Chemistry, University of Guilan, Rasht, Iran Department of Chemistry, Rasht Branch, Islamic Azad University, Rasht, Iran
ARTICLE INFO
ABSTRACT
Keywords: Silica Core-shell Photocatalysis Lanthanum MCM-41 Selenium dioxide
A synthesis method was developed to produce uniform micro-size spheres of silica coated with MCM-41 mesoporous shell for preparation of SiO2@MCM-41 core-shell. Lanthanum ion and selenium dioxide, were introduced separately and together into this core-shell structure. The SiO2@MCM-41, SiO2@SeO2-MCM-41, La–SiO2@MCM-41 and La–SiO2@SeO2-MCM-41core-shells were prepared. A range of reliable instruments and methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Barrett-Joyner-Halenda (BJH), DLS (dynamic light scattering) and energy-dispersive X-ray spectroscopy (EDS) were employed for characterization of the fabricated core-shells. It was found that presence of La in the core and SeO2 in the mesoporous shell, generate very efficient photocatalytic activity of the materials. The prepared photocatalysts were used for degradation of 2,4-dichlorophenol (DCP) as a model compound. The obtained results showed the photocatalytic efficiency was increased in the order of SiO2@MCM-41 (5%) < SiO2@SeO2-MCM-41 (37%) < La–SiO2@MCM-41 (57%) < La–SiO2@SeO2-MCM-41 (87%). The optimized photocatalyst was applied in some real aquatic media including river water and well water sources, and some operational parameters including the effect of catalyst loading, initial concentration of DCP, nominal power of light source etc. on final yield of photodegradation were also investigated. On the other hand antibacterial properties of the prepared substrates were investigated and the results showed effective antibacterial activity of La–SiO2@SeO2-MCM-41 against streptococcus iniae and aeromonas hydrophila.
1. Introduction Inducing various desirable characteristics into just one synthesized particle sometime seems impossible because of the simple fact that some qualities cannot be accumulated in only one structure. During last three decades, many efforts have been devoted to develop hybrid nanoand micro-materials that possess two or more features arose from specific materials or precursors. Core-shells are structures that have been devoted to accumulate various materials and characteristics into just one single particle [1–5]. Fascinating applications of core-shells are due to their potential use in various fields like catalysis, industrial and biomedical applications [6–8]. The core-shell composites and structures can be synthesized with different sizes and shapes of core and shell thickness and various surface morphologies [9]. They may be spherical [10], eccentric [11] or tubular [12] in shape. They could be tuned for special purposes ∗
depending on their size and shape. Whenever the surface of the particles is modified by functional groups or various molecules and/or coated with a thin layer of other materials (with different components), they may develop various enhanced properties compared to the nonfunctionalized uncoated particles [13,14]. Developing the new types of efficient photocatalysts that are active under visible light, is an attractive research area [15]. Semiconductors like SnO2 [16], BiVO4 [17], SeO2 [18] and ZnS [19–21] seem to be cheap and reliable as conventional TiO2 but they need to be modified by various methods such as applying metallic or non-metallic doping species to be active under visible light. Some comprehensive reviews focused on the application of various types of photocatalysts in water and waste water treatment were recently published [22–24]. One of the most common approaches to combine mesoporous materials with other species, especially catalysts, is by impregnation procedure. It is usually accomplished via dispersing both compounds or their precursors in
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Esmaeili), [email protected] (M.A. Zanjanchi).
∗∗
https://doi.org/10.1016/j.micromeso.2019.109714 Received 3 July 2019; Received in revised form 15 August 2019; Accepted 8 September 2019 Available online 09 September 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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water or another solvent followed by evaporation or thermal treatment [25–27]. To the best of our knowledge, there is no report on combinations of silica and selenium oxide into core-shell structures in the literature. There are scientific reports on combination of SeO2 with TiO2 or other well known photocatalysts (see Table 3 in this work). In this paper modification of SeO2 itself and combination with effective substrates (instead of other photocatalysts) was tried. The various core-shells including SiO2@MCM-41, SiO2@SeO2-MCM-41, La–SiO2@MCM-41 and La–SiO2@SeO2-MCM-41 structures were synthesized and their capability in photodegradation of DCP were invetsigated. The capabilities of these materials as antibacterials were studied as well.
Table 1 Surface area values of the prepared substrates in current study. Substrate
BET surface area (m2/g)
SiO2 La–SiO2 SeO2 powdera MCM-41 SeO2- MCM-41 SiO2@MCM-41 SiO2@SeO2- MCM-41 La–SiO2@MCM-41 La–SiO2@SeO2-MCM-41
150 200 10 1350 291 1078 278 358 263
a
2. Experimental
As purchased.
0.05 La2O3: 19.5 H2O. The suspension was stirred for 6 h at r.t. The resulted precipitate was centrifuged and washed with ethanol three times and dried overnight at r.t.
2.1. Materials All the precursors and materials including tetraethyl orthosilicate (SiC8H20O4, TEOS), ammonia solution (28 % wt.), ethanol (C2H5OH), cetyl trimethyl ammonium bromide (C19H42BrN, CTAB), lanthanum (III) nitrate hexahydrate (La(NO3)3·6H2O), selenium dioxide (SeO2), 2,4-dichlorophenol (DCP) and ethylamine (C2H5NH2) were purchased from Merck (Darmstadt, Germany) and Fluka (Buchs, Switzerland) and were used without any further purification. Muller–Hinton Agar was purchased from Sigma-Aldrich (Milwaukee, WI, USA), and the chloramphenicol containing disk was purchased from Tav Arisan Innovators (TANA, Tehran, Iran) and it contained 5 μg of antibiotic compound.
2.3.3. Depositing SeO2-MCM-41 on La–SiO2 core In order to synthesize SeO2-MCM-41 shell on the prepared La–SiO2 core, 500 mg of La–SiO2 particles were dispersed in distilled water (250 ml) by ultrasonication. A solution of 200 ml H2O, 150 ml ethanol, 0.75 g CTAB and 2.84 ml NH4OH (25% wt.) was also prepared and stirred for 10 min. These two solutions were mixed together. Then, 0.75 ml of TEOS and 0.3 g of SeO2 were added and stirred for 24 h at r.t. The resulted white powder was centrifuged, rinsed and dried at 80 °C for 24 h. The dried powder was calcined at 550 °C for 6 h. The same process was used to prepare other substrates listed in Table 1, except some ingredients were omitted or replaced in some cases.
2.2. Characterization The prepared samples were analyzed with standard characterization methods. The XRD patterns of the synthesized structures were recorded by a Philips PW1840 X-ray diffractometer (Netherlands) with Cu Kα radiation (λ = 0.15418 nm). Sibata SA-1100 specific surface area analyzer (Japan) was used to determine the specific surface area of the prepared samples. VEGA TESCAN (Oxford Instruments, UK) energy dispersive X-ray spectroscopy (EDS) was utilized to determine the quantity of the elements in the prepared structures. The size and morphology of the structures were analyzed by a MIRA3-TESCAN-XMU (Oxford Instruments, UK) scanning electron microscope (SEM). A Philips CM-10 (Netherlands) transmission electron microscopy (TEM) was applied to obtain the TEM micrographs. Shimadzu UV-2100 spectrophotometer (Japan) was used to determine the concentration of DCP in aquatic media. N2 adsorption/desorption plots were obtained using a PHS-1020 (PHSCHINA) system (China). Fourier-transform infrared (FTIR) spectra were recorded with Shimadzu FT-IR spectrometer Model 8400 (Japan). The hydrodynamic diameter of particles was investigated by dynamic light scattering (DLS) instrument (Microtrac, model: Nanoflex, USA). Total organic carbon (TOC) was measured using a Shimadzu TOC-L series analyzer.
2.4. Photocatalytic experiments For photocatalytic studies 100 ml of aqueous solutions containing 40 mg/l of DCP was added to a 250 ml beaker and 30 mg of the photocatalyst was added too. The beaker was placed in a continuously ventilated chamber under light source and A 100 Watt commercial tungsten lamp was used as visible light source. A special chamber was designed equipped with fans and a glass spacer in order to control temperature and prevent the solution from adsorbing extra heat radiating from lamp (Fig. S1). The suspension was magnetically stirred before and during irradiation. The distance between the lamp and the surface of solution was about 20 cm. Prior to irradiation, the sample was stirred in dark for 15 min to establish an adsorption-desorption equilibrium between the photocatalyst and the solution. After irradiation for a period of time and isolating the photocatalyst by centrifuge, the concentration of remained DCP in sample solution was measured by UV–Vis spectrophotometer. The calibration curve has been plotted for DCP at wavelength of 285 nm. The absorbance of different standard solutions of DCP from 0.625 to 40 mg/l were recorded at 285 nm for plotting the calibration curve. Fig. S2 shows the UV–Vis absorption spectra of DCP in various concentrations and the resulting calibration curve. The pH of the solutions was measured before starting the catalytic tests. It was mild acidic (around 5.5) in all of the experiments. This pH was not altered considerably during the experiment progress. But it was slightly more acidic at the end of the experiments (around pH = 5.0).
2.3. Preparation of MCM-41 mesoporous silica core-shells 2.3.1. Synthesis of SiO2 core The SiO2 and La–SiO2 cores were synthesized with little modification in the procedure described previously [28]. TEOS (as silica source) was added to a mixture of ethanol and aqueous ammonia to prepare a solution with the TEOS: EtOH: NH4OH: H2O molar ratio as 1.0 : 70.1: 2.1 : 19.5. The suspension was stirred for 6 h at r.t. The resulted precipitate was centrifuged and washed with ethanol three times and dried overnight at r.t.
2.5. Antibacterial study Antibacterial activity of La–SiO2@SeO2-MCM-41 particles was evaluated against gram-positive streptococcus iniae and gram-negative aeromonas hydrophila. Briefly, a suspension of each bacterium with the approximate cell populations of 1.5 × 108 CFU ml−1 was prepared in distilled water and inoculated uniformly onto the surface of Muller–Hinton Agar medium using sterile cotton swabs. Wells of approximately 7 mm in diameter were bored using a well cutter. 20 μl of
2.3.2. Synthesis of La–SiO2 core In order to synthesize La–SiO2 core, 0.05 mol of La(NO3)3·6H2O was add to the above mixture so that the nominal molar ratios including lanthanum source were as follows: 1 TEOS: 70.1 EtOH: 2.1 NH4OH: 2
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Fig. 1. (A) Low-angle XRD patterns of prepared substrates and (B) High-angle XRD patterns of prepared substrates.
suspensions containing 1 mg ml−1 of synthesized particles (in dimethyl sulfoxide) were added to the wells. In order to optimize the bacterial growth, the plates were incubated at 37 °C for 24 h. The assay was performed in triplicates and the antibacterial activity of each sample was determined by measuring the diameter of the zone of inhibition (ZOI) formed around the wells. Chloramphenicol was used as a positive control.
3. Results and discussion 3.1. Characterization of synthesized materials In order to inspect the structure of synthesized substrates, XRD patterns were obtained at low (2ϴ = 1-10°) and high (2ϴ = 10-80°) angles. Fig. 1 (A) shows the XRD patterns recorded at low angels for the 3
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Fig. 3. TEM micrograph of La–SiO2@SeO2-MCM-41 core-shells.
SEM micrographs of the different synthesized core-shell structures are shown in Fig. 2. The most obvious morphology is uniform micro-spheres. The average diameters of these micro-spheres were 200–250 nm. TEM micrograph of La–SiO2@SeO2-MCM-41 is shown in Fig. 3 which is in accordance with SEM micrographs in term of size and uniformity. Since Major component of both core and shells is SiO2 there is no obvious border between cores and shell in taken TEM photos and the thickness of shell cannot be observed, but in a controlled synthesis procedure the net weight of initial cores and final core-shells were measured by an analytic laboratory balance and it was calculated that almost 50% of final core-shell is made of shell (here mesoporous MCM-41). The hydrodynamic diameter of La–SiO2@SeO2-MCM-41 particles was investigated by DLS instrument (Fig. 4). The average hydrodynamic diameters obtained for the particles via DLS technique was around 350 nm. Non-negligible difference between TEM and DLS results (200 and 350 nm for particle diameters, respectively) can be assigned to this fact that the DLS data shows the hydrodynamic diameters. According to these results, the dispersed synthesized particles showed a polydispersity index of 2.258 that reflects a monodisperse sample with the same size. The BET surface areas of the synthesized particles are in the range of 10 m2g-1 (for pure SeO2) and to around 1350 m2g-1 (for pure MCM-41) that are listed in Table 1. Table 1 shows that surface areas of the prepared core-shells are somehow low compared to pure MCM-41. These diminished surface area values can be assigned to contribution of nonmesoporous parts in the matrix of the core-shells. Samples containing La and or Se showed to have much lower surface area than pure MCM-41. The low surface areas of synthesized substrates decrease the role of adsorption process for removal of the probed species and therefore photocatalytic reaction would be the main factor for elimination of DCP. BJH plot of the four main core-shell samples are shown in Fig. 5. Appearance of hysteresis loops between adsorption/desorption branches proves the formation of Type IV mesoporous structure for
Fig. 2. SEM micrographs of prepared substrates.
substrates synthesized in this study. These patterns proved the presence of mesoporous silica in all samples containing MCM-41 substrates. The very sharp peak at 2ϴ = 2.42° can be assigned to ordered hexagonal channels of MCM-41. Although it is worth to mention that the applied method to produce MCM-41 in this study is very sensitive to slight changes of the medium due to introduction of the modifiers [27], yet the XRD patterns of the core-shells containing La and SeO2 show that ordered mesoporous structure is still preserved in the final products. Fig. 2 (B) shows the XRD patterns at higher angles of 10–70°. For the SiO2 and La–SiO2 cores, these patterns clearly show that main substance of the prepared cores is amorphous silica. A very broad feature at around 20 - 30° confirms the presence of amorphous silica. However, in La-containing SiO2, there are two additional reflections at 2ϴ of 28° and 40° which may belong to a cubic phase of La2O3 (based to JCPDS No. 00-040-1284). This means that a part of lanthanum has contributed to La–SiO2 structure as a separate phase. There is no sign of SeO2 in the XRD pattern which can be related to contribution of very small amount of this oxide in the structure or may be due to highly dispersed SeO2 in the structure. It is possible that a considerable amount of added SeO2 to the synthesis solution has been wasted as supernatant and not introduce into the synthetic sol-gel medium. Fig. S3 shows the FT-IR spectrum of the prepared core-shell of La–SiO2@SeO2-MCM-41. The bands around 1055 to 1100 cm−1 and 1210 to 1250 cm−1 can be assinged to the asymetric vibrations of SiO4 units that prove formation of silicate structure. Very broad band around 3450 cm−1 is due to adsorbed water molecules and also surface silanols. The band about 1500 cm−1 is due to La loading and the peak at 2930 cm−1 can show the present of SeO2 in prepared substrates. Fig. S4 shows the elemental analysis of the synthesized substrates. According to the EDS results, the main ingredient of the prepared materials is silicon. Very little amounts of Se were detected in SeO2-containing materials. Presence of La was also detected for the doped substrates. The La/Se molar ratio for the main core-shell (La–SiO2@SeO2-MCM-41) was found to be around 4.44 based on the acquired data from EDS. It is worth to notice that this ratio affects the efficiency of the catalyst for degradation of DCP. In fact, various molar ratios of La/Se were used for synthesizing La–SiO2@SeO2-MCM-41 and the best results in term of photocatalytic degradation of DCP was achieved when La/Se = 4.44.
Fig. 4. DLS pattern of monodispersed La–SiO2@SeO2-MCM-41 particles. 4
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Fig. 5. The nitrogen adsorption/desorption isotherms of prepared substrates.
La–SiO2@SeO2-MCM-41 according to IUPAC description [29]. The shape of BJH plot is determined by the behavior of adsorbate/adsorbent within a varied range of pressure. The very low-pressure part of the isotherm is related to filling up of the micro pores. The low pressure of the isotherm (the very first knee) is related to formation of monolayer of adsorbate (usually nitrogen). Medium part of isotherm is due to formation of multi-layers of adsorbate and the high-pressure end of the plot is formed by condensation of adsorbate molecules inside capillaries [30]. Narrow small hysteresis loop in SiO2@MCM-41 and SiO2@SeO2MCM-41 proves the similarity between adsorption and desorption of probe molecule (N2 with average diameter of ~6 nm) into and from substrate pores/cavities. Expansion of hysteresis loops in La–SiO2@ MCM-41 and La–SiO2@SeO2-MCM-41 shows more interaction between substrate and adsorbate molecules and also difficulty for probe molecule to be evaporated and escaped from the pores and cavities. Elongated hysteresis loop can also be a sign of stronger interaction between adsorbate (here N2 gas) and substrate (here core-shells). A schematic illustration and a depiction of proposed mechanism for the photodegradation of DCP is given in Fig. 6 showing the synthesis steps and photocatalytic process of the La–SiO2@SeO2-MCM-41. It has been suggested in some literature that main steps of photocatalytic degradation of DCP are detachment of chloride substitutions, opening of the aromatic ring, formation of carboxylic acid and CO2, respectively [31].
core-shell structures were studied using 100 ml solution containing 40 mg/l DCP and 30 mg of the catalysts. The process was continued for 90 min of irradiation. All the substrates showed a minimum adsorption tendency and the reduction in DCP concentration was observed after switching on the tungsten lamp. The results (Fig. 7), showed no obvious photocatalytic degradation of DCP using SiO2@MCM-41. According to the results (Fig. 7), DCP degradation progressed upon continuing irradiation. The best photocatalytic activity was observed for La–SiO2@ SeO2-MCM-41. The photodegradation efficiency of DCP is calculated from the following equation:
D% =
C0
C C0
× 100
In which C0 is the initial concentration of DCP, C is the concentration of DCP at the time of t and D% is the photodegradation efficiency in percent. The photocatalytic efficiencies of DCP degradation after 90 min irradiation were obtained in the order of SiO2@MCM-41 (5%) < SiO2@ SeO2-MCM-41 (37%) < La–SiO2@MCM-41 (55%) < La–SiO2@SeO2MCM-41 (87%). It seems that presence of both SeO2 and La is necessary to achieve the highest photocatalytic efficiency. The higher photocatalytic efficiency of La-doped core-shells can be related to the electron-scavenging effect of La3+. Also, there are some reports on the photocatalytic capability of La-dopped metal-oxides that suggest La improves total quantum efficiency for catalysis by preventing the fast electron-hole recombination [32–34]. Therefore, La–SiO2@SeO2-MCM-41 core-shell photocatalyst is known to be the best one among the other synthesized core-shells for photodegradation of DCP. In order to determine the contribution of adsorption phenomenon in the total removal efficiency, all the photocatalysts were
3.2. Photocatalytic activity The photodegradation of DCP using the synthesized SiO2@MCM-41, SiO2@SeO2-MCM-41, La–SiO2@MCM-41 and La–SiO2@SeO2-MCM-41 5
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Fig. 6. A schematic representation for synthesis of the La–SiO2@SeO2-MCM-41 micro-spheres and their application in DCP photodegradation.
onto MCM-41 surface. As mentioned so far, several La–SiO2@SeO2-MCM-41 core-shells with various La/Se molar ratios were synthesized and were studied. It was found that the molar ratio of 4.4 for La/Se shows the best photocatalytic efficiency in degradation of DCP (Fig. S6). In order to consider the effect of DCP concentration on the photodegradation efficiency, 100 ml solution containing DCP concentrations (10, 20, 30 and 40 mg/l) were examined using La–SiO2@SeO2-MCM41. The results showed that the degradation efficiency of 100% for DCP concentrations of 10, 20 and 30 mg/l. Also, decreasing DCP concentration will reduce photodegradation time for 100% removal to 30 min for 10 mg/l, 40 min for 20 mg/l and 75 min for 30 mg/l of DCP. 3.3. Photodegradation of DCP in real samples The capability of the prepared La–SiO2@SeO2-MCM-41 was tested for degradation of DCP in real samples. Therefore, three aqueous samples including one well water (Rasht, Iran) and two river water (Sefid-Rood and Bijar, Rasht, Iran) were collected and examined. DCP content of the samples were measured and no DCP was found. Some of the characteristics of the samples including pH, conductivity, turbidity and total dissolved solids (TDS) were measured. The results are summarized in Table 2. These water samples aged for about 1 h in order to precipitate large mud particles. Then, they were filtered through Whatman 40 (pore size~ 8 μm) to remove colloids and silt particles. Then, the filtrate was spiked with 40 mg/l of DCP. Three new calibration curves were prepared and the photocatalytic degradation of DCP was investigated in the presence of La–SiO2@SeO2-MCM-41. Table 2
Fig. 7. Photocatalytic activities of prepared substrates in degradation of DCP. Operational conditions were as follow: in separate experiments 30 mg of each substrate were dispersed into 100 ml of aqueous solution of DCP with concentration of 40 mg/L.
exposed to the same concentration of DCP and stirred up to 90 min under total darkness (Fig. S5). Almost no adsorption was observed for any of the studied substrates toward DCP. Although SiO2 exhibit surface groups like SiOH which partly dissociate in aqueous solution depending on pH, apparently there is no considerable adsorptive behavior for DCP 6
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Table 2 Performance of La–SiO2@SeO2-MCM-41 for photodegradation of DCP in real samples, Operational conditions were as follow: in separate experiments 30 mg of each substrate were dispersed into 100 ml of aqueous solution of DCP with concentration of 40 mg/L. Aquatic medium
Deionized water Sefid-Rood River Bijar River Well Water a b c
Photocatalytic efficiency (%)c
Initial characteristics of aquatic medium before introduction of DCP and photocatalyst pH
Conductivity (μS/cm)
Turbidity (NTUa)
TDSb (mg/L)
~6.8 8.10 7.95 7.05
~0 1151 221 849
~0 0.39 3.3 27.3
~0 531 102 401
85% 76% 74% 56%
Nephelometric Turbidity Unit. Total dissolved solids. See section 3.2 of the current work.
Table 3 Comparison of the recently published works using Se- or SeO2-containing catalysts with the present work. Catalyst
Pollutant
Initial Concentration, Volume
Light Source
Time of Irradiation
Efficiency of Catalyst
Reference
Se-doped Bi2O3 Se-doped TiO2
methylene blue 4-nitrophenol
10 mg/L, 250 ml 1.4 mg/L, 100 ml
12 h 120 min
95% 87%
[40] [41]
SeO2/TiO2 Se-doped ZnS MoSe2
sunset yellow trypan blue Methylene Blue Rhodamine B Methyl Orange 2, 4- dichlorophenol 2, 4- dichlorophenol 2, 4- dichlorophenol 2, 4- dichlorophenol
0.04 mg/L, 50 ml 15 mg/L, 50 ml 15 mg/L, 15 ml 15 mg/L, 15 ml 15 mg/L, 15 ml 10 mg/L, 100 ml 20 mg/L, 100 ml 30 mg/L, 100 ml 40 mg/L, 100 ml
300 Watt Xenon Lamp 5 × 8 Watt Blacklight Fluorescent Lamps Sunlight Sunlight 500 Watt Xenon Lamp 500 Watt Xenon Lamp 500 Watt Xenon Lamp 100 Watt Tungsten Lamp 100 Watt Tungsten Lamp 100 Watt Tungsten Lamp 100 Watt Tungsten Lamp
90 min 180 min 180 min 180 min 180 min 30 min 40 min 75 min 90 min
90% 90% 90% 90% 90% 100% 100% 100% 85%
[42] [43] [44]
La2O3/SiO2@SeO2-MCM-41 Core-shell
This This This This
study study study study
shows a comparison between the results of photocatalytic degradation of DCP in deionized water and in three real samples. It clearly shows the potential of the prepared photocatalyst for real samples. Slightly lower efficiency of La–SiO2@SeO2-MCM-41 in real samples might be because of various unknown organic compounds or micro-organisms that might occupy active catalytic sites of the photocatalyst. Different initial characteristics of aquatic media in terms of pH, turbidity, dissolved solids and etc. also can affect the efficiency of the photocatalyst. Apparently high turbidity and TDS are the most effective parameters on decreasing the final efficiency. Fig. S7 shows changes of UV spectra and concentration of DCP vs irradiation time for aforementioned photocatalytic experiments in real samples.
related to the electric charge of DCP itself and the hydrolysis of the surface of photocatalyst in various pH values. Also alkaline pH resulted in very noticeable changes in spectral behavior of DCP.
3.4. Recyclability of La–SiO2@SeO2-MCM-41
3.5.3. Effect of initial concentration of DCP 100 ml of various concentrations of DCP were tested in presence of 30 mg of La–SiO2@SeO2-MCM-41. The results are shown in Table 3. All concentrations less than 30 mg/L of DCP were completely degraded in less than 75 min.
3.5.2. Effect of loaded amounts of photocatalyst Various amounts of La–SiO2@SeO2-MCM-41 from 10 to 100 mg were added to 100 ml of DCP (40 mg/L) and the dosage effect of photocatalyst was investigated. Low amounts of photocatalyst resulted in less photodegradation and also dosage more than 30 mg/100 ml of DCP (40 mg/L) showed less efficiency that can be because of shadowing effect of catalyst powder that prevent the light from penetrating the solution. All results are shown in Fig. S10.
The recyclability of the catalyst is one of the important parameters in heterogeneous processes. In order to investigate the recyclability of our photocatalyst, subsequent to each photodegradation experiment, La–SiO2@SeO2-MCM-41 was separated by centrifuge, washed with hot distilled water, sonicated in ethanol and dried at 80 °C for 2 h. Then, the recovered catalyst was weighted by an analytical balance and used for the next photodegradation experiment. This process was repeated for at least 5 times. After 5 cycle of recovery, the total efficiency in photodegradation of DCP dropped less than 50% compared to fresh substrate (Fig. S8).
3.5.4. Effect of nominal power of light source In current work the conventional tungsten lamps from a local company (Pars Shahab Lamp Co.) were used as light sources. The same type of lamp with different nominal power was purchased (Fig. S11). The lamp with 40 Watt of nominal power was almost ineffective in photodegradation of DCP. By increasing the power of lamps the total photocatalytic efficiency increased but 200 Watt lamp was just slightly better than the conventional 100 Watt lamp plus the 200 Watt light source produced a lot of heat that the ventilated chamber of reaction was not capable of exhausting it very well. This experiment showed that to observe a distinguished photocatalytic reaction at least a nominal 60 Watt tungsten lamp is needed. In order to determine the extent of decomposition of DCP, total organic carbon (TOC) were measured after each experiment (lables of each point in Fig. S11.) Increasing the power of lamp actually decreased the amount of TOC that can be assigned to decomposition of DCP. Ratio of C/C0 is also proportional to TOC values
3.5. Operational parameters and their effect on photocatalytic efficiency In order to investigate the influence of pH, loading of photocatalyst (dosage), initial concentration of DCP, nominal power of light source and salt effect (ionic strength of solution) on photocatalytic activity of La–SiO2@SeO2-MCM-41 some experiments were designed. 3.5.1. Effect of pH The best operational pH was found to be 5.5 and lower or higher pH values diminished the photocatalytic efficiency (Fig. S9). This can be 7
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Fig. 8. Antibacterial activity of prepared substrates against different pathogenic bacteria.
which prove that UV spectra can be used as a reliable tool to determine the extent of decomposition.
3.8. Comparison with other catalysts Table 3 summarizes details of some recent studies using SeO2 or Se containing supports and compares the results with those of the current study. As shown in Table 3, the present study shows some advantages compared to the previously reported works. These include: (1) the photocatalyst showed good efficiency for higher sample volume and concentration with a consumption of low dose of photocatalyst (0.03 g in 100 ml solution), (2) higher efficiency for higher concentrations of DCP (100% DCP degradation for 30 mg/l of DCP within 75 min), (3) The shorter time for nearly complete degradation, (4) use of low-energy tungsten lamp as source of irradiation.
3.6. Salt effect Photocatalytic experiment was executed in presence of various concentrations of NaCl as a typical species present in many real water media. Salts can alter the ionic strength of solution and changes the behavior of both targeted pollutant and catalyst. Results are shown in Fig. S12. By increasing the concentration of NaCl the efficiency of photodegradion were declined. Rajabi et al. [35] mentioned some reasons for this phenomenon that are related to the presence of Cl− anions; first, competition between Cl− and pollutant (here DCP) to reach the surface of the catalyst. Second, Cl− can act as an h+ trap and prevent it from engaging in formation of reactive radicals.
4. Conclusions La–SiO2@SeO2-MCM-41 core-shells were synthesized via simple solgel method. Distribution of SeO2 into a mesoporous matrix resulted in a considerable photocatalytic activity. Introduction of La3+ to the coreshell improved the photocatalytic efficiency which may be assigned to electron-scavenging effect of La3+. Our comprehensive experimental study proved that presence of both SeO2 and La3+ lead to achieve a highly efficient photocatalyst based on SiO2 core and MCM- 41 shell. Prepared core-shell catalyst showed a reasonable ability in degradation of DCP in real aquatic samples. Also its antibacterial activity was noticeable.
3.7. Antibacterial activity The antibacterial activity of La–SiO2@SeO2-MCM-41 micro-particles against pathogenic bacteria (including gram-positive streptococcus iniae and gram-negative aeromonas hydrophila) were investigated through measuring inhabitation zone diameter (Fig. 8). It is clear that these substrates show certain level of antimicrobial activity against the investigated pathogens. Zone of inhibition (ZOI) for La-containing substrates were wider than the ones without La. In a previous study [36], it was suggested that the penetration of reactive oxygen species (ROS), probably produced on the surface of photocatalysts, onto the cell membrane can cause to damage of cellular proteins and cell death. Furthermore, the obtained results indicated that the streptococcus iniae is the most susceptible bacteria with almost ZOI of 18 mm when was exposed to 20 mg/l concentration of La–SiO2@SeO2-MCM-41 dispersed in dimethyl sulfoxide. Aeromonas hydrophila showed some resistance and ZOI was about 12 mm. The highest microbial resistance to La–SiO2@SeO2-MCM-41 was seen for Staphylococcus aureus. Bacterial cells are surrounded by a complex and multilayer wall which act as their defense against many antibacterial agents. The cell wall of bacteria consists of a thin or thick layer of peptidoglycan, surrounded by a lipopolysaccharide (LPS) membrane which is the main barrier to protect them [36–38]. Any form of weakness in this walls can end up in penetration of reactive agents (here reactive oxygen species) and consequent damages. Higher susceptibility of gram–negative bacteria to metal oxide NPs has been reported for CuO NPs, previously [39].
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis, characterization and photocatalytic studies of MCM-41 mesoporous silica core-shells doped with selenium oxide and lanthanum ions
T
Samaneh Esmaeilia,∗, Mohammad Ali Zanjanchia,∗∗, Hossein Golmojdeha, Shahab Shariatib a b
Faculty of Science, Department of Chemistry, University of Guilan, Rasht, Iran Department of Chemistry, Rasht Branch, Islamic Azad University, Rasht, Iran
ARTICLE INFO
ABSTRACT
Keywords: Silica Core-shell Photocatalysis Lanthanum MCM-41 Selenium dioxide
A synthesis method was developed to produce uniform micro-size spheres of silica coated with MCM-41 mesoporous shell for preparation of SiO2@MCM-41 core-shell. Lanthanum ion and selenium dioxide, were introduced separately and together into this core-shell structure. The SiO2@MCM-41, SiO2@SeO2-MCM-41, La–SiO2@MCM-41 and La–SiO2@SeO2-MCM-41core-shells were prepared. A range of reliable instruments and methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Barrett-Joyner-Halenda (BJH), DLS (dynamic light scattering) and energy-dispersive X-ray spectroscopy (EDS) were employed for characterization of the fabricated core-shells. It was found that presence of La in the core and SeO2 in the mesoporous shell, generate very efficient photocatalytic activity of the materials. The prepared photocatalysts were used for degradation of 2,4-dichlorophenol (DCP) as a model compound. The obtained results showed the photocatalytic efficiency was increased in the order of SiO2@MCM-41 (5%) < SiO2@SeO2-MCM-41 (37%) < La–SiO2@MCM-41 (57%) < La–SiO2@SeO2-MCM-41 (87%). The optimized photocatalyst was applied in some real aquatic media including river water and well water sources, and some operational parameters including the effect of catalyst loading, initial concentration of DCP, nominal power of light source etc. on final yield of photodegradation were also investigated. On the other hand antibacterial properties of the prepared substrates were investigated and the results showed effective antibacterial activity of La–SiO2@SeO2-MCM-41 against streptococcus iniae and aeromonas hydrophila.
1. Introduction Inducing various desirable characteristics into just one synthesized particle sometime seems impossible because of the simple fact that some qualities cannot be accumulated in only one structure. During last three decades, many efforts have been devoted to develop hybrid nanoand micro-materials that possess two or more features arose from specific materials or precursors. Core-shells are structures that have been devoted to accumulate various materials and characteristics into just one single particle [1–5]. Fascinating applications of core-shells are due to their potential use in various fields like catalysis, industrial and biomedical applications [6–8]. The core-shell composites and structures can be synthesized with different sizes and shapes of core and shell thickness and various surface morphologies [9]. They may be spherical [10], eccentric [11] or tubular [12] in shape. They could be tuned for special purposes ∗
depending on their size and shape. Whenever the surface of the particles is modified by functional groups or various molecules and/or coated with a thin layer of other materials (with different components), they may develop various enhanced properties compared to the nonfunctionalized uncoated particles [13,14]. Developing the new types of efficient photocatalysts that are active under visible light, is an attractive research area [15]. Semiconductors like SnO2 [16], BiVO4 [17], SeO2 [18] and ZnS [19–21] seem to be cheap and reliable as conventional TiO2 but they need to be modified by various methods such as applying metallic or non-metallic doping species to be active under visible light. Some comprehensive reviews focused on the application of various types of photocatalysts in water and waste water treatment were recently published [22–24]. One of the most common approaches to combine mesoporous materials with other species, especially catalysts, is by impregnation procedure. It is usually accomplished via dispersing both compounds or their precursors in
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Esmaeili), [email protected] (M.A. Zanjanchi).
∗∗
https://doi.org/10.1016/j.micromeso.2019.109714 Received 3 July 2019; Received in revised form 15 August 2019; Accepted 8 September 2019 Available online 09 September 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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water or another solvent followed by evaporation or thermal treatment [25–27]. To the best of our knowledge, there is no report on combinations of silica and selenium oxide into core-shell structures in the literature. There are scientific reports on combination of SeO2 with TiO2 or other well known photocatalysts (see Table 3 in this work). In this paper modification of SeO2 itself and combination with effective substrates (instead of other photocatalysts) was tried. The various core-shells including SiO2@MCM-41, SiO2@SeO2-MCM-41, La–SiO2@MCM-41 and La–SiO2@SeO2-MCM-41 structures were synthesized and their capability in photodegradation of DCP were invetsigated. The capabilities of these materials as antibacterials were studied as well.
Table 1 Surface area values of the prepared substrates in current study. Substrate
BET surface area (m2/g)
SiO2 La–SiO2 SeO2 powdera MCM-41 SeO2- MCM-41 SiO2@MCM-41 SiO2@SeO2- MCM-41 La–SiO2@MCM-41 La–SiO2@SeO2-MCM-41
150 200 10 1350 291 1078 278 358 263
a
2. Experimental
As purchased.
0.05 La2O3: 19.5 H2O. The suspension was stirred for 6 h at r.t. The resulted precipitate was centrifuged and washed with ethanol three times and dried overnight at r.t.
2.1. Materials All the precursors and materials including tetraethyl orthosilicate (SiC8H20O4, TEOS), ammonia solution (28 % wt.), ethanol (C2H5OH), cetyl trimethyl ammonium bromide (C19H42BrN, CTAB), lanthanum (III) nitrate hexahydrate (La(NO3)3·6H2O), selenium dioxide (SeO2), 2,4-dichlorophenol (DCP) and ethylamine (C2H5NH2) were purchased from Merck (Darmstadt, Germany) and Fluka (Buchs, Switzerland) and were used without any further purification. Muller–Hinton Agar was purchased from Sigma-Aldrich (Milwaukee, WI, USA), and the chloramphenicol containing disk was purchased from Tav Arisan Innovators (TANA, Tehran, Iran) and it contained 5 μg of antibiotic compound.
2.3.3. Depositing SeO2-MCM-41 on La–SiO2 core In order to synthesize SeO2-MCM-41 shell on the prepared La–SiO2 core, 500 mg of La–SiO2 particles were dispersed in distilled water (250 ml) by ultrasonication. A solution of 200 ml H2O, 150 ml ethanol, 0.75 g CTAB and 2.84 ml NH4OH (25% wt.) was also prepared and stirred for 10 min. These two solutions were mixed together. Then, 0.75 ml of TEOS and 0.3 g of SeO2 were added and stirred for 24 h at r.t. The resulted white powder was centrifuged, rinsed and dried at 80 °C for 24 h. The dried powder was calcined at 550 °C for 6 h. The same process was used to prepare other substrates listed in Table 1, except some ingredients were omitted or replaced in some cases.
2.2. Characterization The prepared samples were analyzed with standard characterization methods. The XRD patterns of the synthesized structures were recorded by a Philips PW1840 X-ray diffractometer (Netherlands) with Cu Kα radiation (λ = 0.15418 nm). Sibata SA-1100 specific surface area analyzer (Japan) was used to determine the specific surface area of the prepared samples. VEGA TESCAN (Oxford Instruments, UK) energy dispersive X-ray spectroscopy (EDS) was utilized to determine the quantity of the elements in the prepared structures. The size and morphology of the structures were analyzed by a MIRA3-TESCAN-XMU (Oxford Instruments, UK) scanning electron microscope (SEM). A Philips CM-10 (Netherlands) transmission electron microscopy (TEM) was applied to obtain the TEM micrographs. Shimadzu UV-2100 spectrophotometer (Japan) was used to determine the concentration of DCP in aquatic media. N2 adsorption/desorption plots were obtained using a PHS-1020 (PHSCHINA) system (China). Fourier-transform infrared (FTIR) spectra were recorded with Shimadzu FT-IR spectrometer Model 8400 (Japan). The hydrodynamic diameter of particles was investigated by dynamic light scattering (DLS) instrument (Microtrac, model: Nanoflex, USA). Total organic carbon (TOC) was measured using a Shimadzu TOC-L series analyzer.
2.4. Photocatalytic experiments For photocatalytic studies 100 ml of aqueous solutions containing 40 mg/l of DCP was added to a 250 ml beaker and 30 mg of the photocatalyst was added too. The beaker was placed in a continuously ventilated chamber under light source and A 100 Watt commercial tungsten lamp was used as visible light source. A special chamber was designed equipped with fans and a glass spacer in order to control temperature and prevent the solution from adsorbing extra heat radiating from lamp (Fig. S1). The suspension was magnetically stirred before and during irradiation. The distance between the lamp and the surface of solution was about 20 cm. Prior to irradiation, the sample was stirred in dark for 15 min to establish an adsorption-desorption equilibrium between the photocatalyst and the solution. After irradiation for a period of time and isolating the photocatalyst by centrifuge, the concentration of remained DCP in sample solution was measured by UV–Vis spectrophotometer. The calibration curve has been plotted for DCP at wavelength of 285 nm. The absorbance of different standard solutions of DCP from 0.625 to 40 mg/l were recorded at 285 nm for plotting the calibration curve. Fig. S2 shows the UV–Vis absorption spectra of DCP in various concentrations and the resulting calibration curve. The pH of the solutions was measured before starting the catalytic tests. It was mild acidic (around 5.5) in all of the experiments. This pH was not altered considerably during the experiment progress. But it was slightly more acidic at the end of the experiments (around pH = 5.0).
2.3. Preparation of MCM-41 mesoporous silica core-shells 2.3.1. Synthesis of SiO2 core The SiO2 and La–SiO2 cores were synthesized with little modification in the procedure described previously [28]. TEOS (as silica source) was added to a mixture of ethanol and aqueous ammonia to prepare a solution with the TEOS: EtOH: NH4OH: H2O molar ratio as 1.0 : 70.1: 2.1 : 19.5. The suspension was stirred for 6 h at r.t. The resulted precipitate was centrifuged and washed with ethanol three times and dried overnight at r.t.
2.5. Antibacterial study Antibacterial activity of La–SiO2@SeO2-MCM-41 particles was evaluated against gram-positive streptococcus iniae and gram-negative aeromonas hydrophila. Briefly, a suspension of each bacterium with the approximate cell populations of 1.5 × 108 CFU ml−1 was prepared in distilled water and inoculated uniformly onto the surface of Muller–Hinton Agar medium using sterile cotton swabs. Wells of approximately 7 mm in diameter were bored using a well cutter. 20 μl of
2.3.2. Synthesis of La–SiO2 core In order to synthesize La–SiO2 core, 0.05 mol of La(NO3)3·6H2O was add to the above mixture so that the nominal molar ratios including lanthanum source were as follows: 1 TEOS: 70.1 EtOH: 2.1 NH4OH: 2
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Fig. 1. (A) Low-angle XRD patterns of prepared substrates and (B) High-angle XRD patterns of prepared substrates.
suspensions containing 1 mg ml−1 of synthesized particles (in dimethyl sulfoxide) were added to the wells. In order to optimize the bacterial growth, the plates were incubated at 37 °C for 24 h. The assay was performed in triplicates and the antibacterial activity of each sample was determined by measuring the diameter of the zone of inhibition (ZOI) formed around the wells. Chloramphenicol was used as a positive control.
3. Results and discussion 3.1. Characterization of synthesized materials In order to inspect the structure of synthesized substrates, XRD patterns were obtained at low (2ϴ = 1-10°) and high (2ϴ = 10-80°) angles. Fig. 1 (A) shows the XRD patterns recorded at low angels for the 3
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Fig. 3. TEM micrograph of La–SiO2@SeO2-MCM-41 core-shells.
SEM micrographs of the different synthesized core-shell structures are shown in Fig. 2. The most obvious morphology is uniform micro-spheres. The average diameters of these micro-spheres were 200–250 nm. TEM micrograph of La–SiO2@SeO2-MCM-41 is shown in Fig. 3 which is in accordance with SEM micrographs in term of size and uniformity. Since Major component of both core and shells is SiO2 there is no obvious border between cores and shell in taken TEM photos and the thickness of shell cannot be observed, but in a controlled synthesis procedure the net weight of initial cores and final core-shells were measured by an analytic laboratory balance and it was calculated that almost 50% of final core-shell is made of shell (here mesoporous MCM-41). The hydrodynamic diameter of La–SiO2@SeO2-MCM-41 particles was investigated by DLS instrument (Fig. 4). The average hydrodynamic diameters obtained for the particles via DLS technique was around 350 nm. Non-negligible difference between TEM and DLS results (200 and 350 nm for particle diameters, respectively) can be assigned to this fact that the DLS data shows the hydrodynamic diameters. According to these results, the dispersed synthesized particles showed a polydispersity index of 2.258 that reflects a monodisperse sample with the same size. The BET surface areas of the synthesized particles are in the range of 10 m2g-1 (for pure SeO2) and to around 1350 m2g-1 (for pure MCM-41) that are listed in Table 1. Table 1 shows that surface areas of the prepared core-shells are somehow low compared to pure MCM-41. These diminished surface area values can be assigned to contribution of nonmesoporous parts in the matrix of the core-shells. Samples containing La and or Se showed to have much lower surface area than pure MCM-41. The low surface areas of synthesized substrates decrease the role of adsorption process for removal of the probed species and therefore photocatalytic reaction would be the main factor for elimination of DCP. BJH plot of the four main core-shell samples are shown in Fig. 5. Appearance of hysteresis loops between adsorption/desorption branches proves the formation of Type IV mesoporous structure for
Fig. 2. SEM micrographs of prepared substrates.
substrates synthesized in this study. These patterns proved the presence of mesoporous silica in all samples containing MCM-41 substrates. The very sharp peak at 2ϴ = 2.42° can be assigned to ordered hexagonal channels of MCM-41. Although it is worth to mention that the applied method to produce MCM-41 in this study is very sensitive to slight changes of the medium due to introduction of the modifiers [27], yet the XRD patterns of the core-shells containing La and SeO2 show that ordered mesoporous structure is still preserved in the final products. Fig. 2 (B) shows the XRD patterns at higher angles of 10–70°. For the SiO2 and La–SiO2 cores, these patterns clearly show that main substance of the prepared cores is amorphous silica. A very broad feature at around 20 - 30° confirms the presence of amorphous silica. However, in La-containing SiO2, there are two additional reflections at 2ϴ of 28° and 40° which may belong to a cubic phase of La2O3 (based to JCPDS No. 00-040-1284). This means that a part of lanthanum has contributed to La–SiO2 structure as a separate phase. There is no sign of SeO2 in the XRD pattern which can be related to contribution of very small amount of this oxide in the structure or may be due to highly dispersed SeO2 in the structure. It is possible that a considerable amount of added SeO2 to the synthesis solution has been wasted as supernatant and not introduce into the synthetic sol-gel medium. Fig. S3 shows the FT-IR spectrum of the prepared core-shell of La–SiO2@SeO2-MCM-41. The bands around 1055 to 1100 cm−1 and 1210 to 1250 cm−1 can be assinged to the asymetric vibrations of SiO4 units that prove formation of silicate structure. Very broad band around 3450 cm−1 is due to adsorbed water molecules and also surface silanols. The band about 1500 cm−1 is due to La loading and the peak at 2930 cm−1 can show the present of SeO2 in prepared substrates. Fig. S4 shows the elemental analysis of the synthesized substrates. According to the EDS results, the main ingredient of the prepared materials is silicon. Very little amounts of Se were detected in SeO2-containing materials. Presence of La was also detected for the doped substrates. The La/Se molar ratio for the main core-shell (La–SiO2@SeO2-MCM-41) was found to be around 4.44 based on the acquired data from EDS. It is worth to notice that this ratio affects the efficiency of the catalyst for degradation of DCP. In fact, various molar ratios of La/Se were used for synthesizing La–SiO2@SeO2-MCM-41 and the best results in term of photocatalytic degradation of DCP was achieved when La/Se = 4.44.
Fig. 4. DLS pattern of monodispersed La–SiO2@SeO2-MCM-41 particles. 4
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Fig. 5. The nitrogen adsorption/desorption isotherms of prepared substrates.
La–SiO2@SeO2-MCM-41 according to IUPAC description [29]. The shape of BJH plot is determined by the behavior of adsorbate/adsorbent within a varied range of pressure. The very low-pressure part of the isotherm is related to filling up of the micro pores. The low pressure of the isotherm (the very first knee) is related to formation of monolayer of adsorbate (usually nitrogen). Medium part of isotherm is due to formation of multi-layers of adsorbate and the high-pressure end of the plot is formed by condensation of adsorbate molecules inside capillaries [30]. Narrow small hysteresis loop in SiO2@MCM-41 and SiO2@SeO2MCM-41 proves the similarity between adsorption and desorption of probe molecule (N2 with average diameter of ~6 nm) into and from substrate pores/cavities. Expansion of hysteresis loops in La–SiO2@ MCM-41 and La–SiO2@SeO2-MCM-41 shows more interaction between substrate and adsorbate molecules and also difficulty for probe molecule to be evaporated and escaped from the pores and cavities. Elongated hysteresis loop can also be a sign of stronger interaction between adsorbate (here N2 gas) and substrate (here core-shells). A schematic illustration and a depiction of proposed mechanism for the photodegradation of DCP is given in Fig. 6 showing the synthesis steps and photocatalytic process of the La–SiO2@SeO2-MCM-41. It has been suggested in some literature that main steps of photocatalytic degradation of DCP are detachment of chloride substitutions, opening of the aromatic ring, formation of carboxylic acid and CO2, respectively [31].
core-shell structures were studied using 100 ml solution containing 40 mg/l DCP and 30 mg of the catalysts. The process was continued for 90 min of irradiation. All the substrates showed a minimum adsorption tendency and the reduction in DCP concentration was observed after switching on the tungsten lamp. The results (Fig. 7), showed no obvious photocatalytic degradation of DCP using SiO2@MCM-41. According to the results (Fig. 7), DCP degradation progressed upon continuing irradiation. The best photocatalytic activity was observed for La–SiO2@ SeO2-MCM-41. The photodegradation efficiency of DCP is calculated from the following equation:
D% =
C0
C C0
× 100
In which C0 is the initial concentration of DCP, C is the concentration of DCP at the time of t and D% is the photodegradation efficiency in percent. The photocatalytic efficiencies of DCP degradation after 90 min irradiation were obtained in the order of SiO2@MCM-41 (5%) < SiO2@ SeO2-MCM-41 (37%) < La–SiO2@MCM-41 (55%) < La–SiO2@SeO2MCM-41 (87%). It seems that presence of both SeO2 and La is necessary to achieve the highest photocatalytic efficiency. The higher photocatalytic efficiency of La-doped core-shells can be related to the electron-scavenging effect of La3+. Also, there are some reports on the photocatalytic capability of La-dopped metal-oxides that suggest La improves total quantum efficiency for catalysis by preventing the fast electron-hole recombination [32–34]. Therefore, La–SiO2@SeO2-MCM-41 core-shell photocatalyst is known to be the best one among the other synthesized core-shells for photodegradation of DCP. In order to determine the contribution of adsorption phenomenon in the total removal efficiency, all the photocatalysts were
3.2. Photocatalytic activity The photodegradation of DCP using the synthesized SiO2@MCM-41, SiO2@SeO2-MCM-41, La–SiO2@MCM-41 and La–SiO2@SeO2-MCM-41 5
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Fig. 6. A schematic representation for synthesis of the La–SiO2@SeO2-MCM-41 micro-spheres and their application in DCP photodegradation.
onto MCM-41 surface. As mentioned so far, several La–SiO2@SeO2-MCM-41 core-shells with various La/Se molar ratios were synthesized and were studied. It was found that the molar ratio of 4.4 for La/Se shows the best photocatalytic efficiency in degradation of DCP (Fig. S6). In order to consider the effect of DCP concentration on the photodegradation efficiency, 100 ml solution containing DCP concentrations (10, 20, 30 and 40 mg/l) were examined using La–SiO2@SeO2-MCM41. The results showed that the degradation efficiency of 100% for DCP concentrations of 10, 20 and 30 mg/l. Also, decreasing DCP concentration will reduce photodegradation time for 100% removal to 30 min for 10 mg/l, 40 min for 20 mg/l and 75 min for 30 mg/l of DCP. 3.3. Photodegradation of DCP in real samples The capability of the prepared La–SiO2@SeO2-MCM-41 was tested for degradation of DCP in real samples. Therefore, three aqueous samples including one well water (Rasht, Iran) and two river water (Sefid-Rood and Bijar, Rasht, Iran) were collected and examined. DCP content of the samples were measured and no DCP was found. Some of the characteristics of the samples including pH, conductivity, turbidity and total dissolved solids (TDS) were measured. The results are summarized in Table 2. These water samples aged for about 1 h in order to precipitate large mud particles. Then, they were filtered through Whatman 40 (pore size~ 8 μm) to remove colloids and silt particles. Then, the filtrate was spiked with 40 mg/l of DCP. Three new calibration curves were prepared and the photocatalytic degradation of DCP was investigated in the presence of La–SiO2@SeO2-MCM-41. Table 2
Fig. 7. Photocatalytic activities of prepared substrates in degradation of DCP. Operational conditions were as follow: in separate experiments 30 mg of each substrate were dispersed into 100 ml of aqueous solution of DCP with concentration of 40 mg/L.
exposed to the same concentration of DCP and stirred up to 90 min under total darkness (Fig. S5). Almost no adsorption was observed for any of the studied substrates toward DCP. Although SiO2 exhibit surface groups like SiOH which partly dissociate in aqueous solution depending on pH, apparently there is no considerable adsorptive behavior for DCP 6
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Table 2 Performance of La–SiO2@SeO2-MCM-41 for photodegradation of DCP in real samples, Operational conditions were as follow: in separate experiments 30 mg of each substrate were dispersed into 100 ml of aqueous solution of DCP with concentration of 40 mg/L. Aquatic medium
Deionized water Sefid-Rood River Bijar River Well Water a b c
Photocatalytic efficiency (%)c
Initial characteristics of aquatic medium before introduction of DCP and photocatalyst pH
Conductivity (μS/cm)
Turbidity (NTUa)
TDSb (mg/L)
~6.8 8.10 7.95 7.05
~0 1151 221 849
~0 0.39 3.3 27.3
~0 531 102 401
85% 76% 74% 56%
Nephelometric Turbidity Unit. Total dissolved solids. See section 3.2 of the current work.
Table 3 Comparison of the recently published works using Se- or SeO2-containing catalysts with the present work. Catalyst
Pollutant
Initial Concentration, Volume
Light Source
Time of Irradiation
Efficiency of Catalyst
Reference
Se-doped Bi2O3 Se-doped TiO2
methylene blue 4-nitrophenol
10 mg/L, 250 ml 1.4 mg/L, 100 ml
12 h 120 min
95% 87%
[40] [41]
SeO2/TiO2 Se-doped ZnS MoSe2
sunset yellow trypan blue Methylene Blue Rhodamine B Methyl Orange 2, 4- dichlorophenol 2, 4- dichlorophenol 2, 4- dichlorophenol 2, 4- dichlorophenol
0.04 mg/L, 50 ml 15 mg/L, 50 ml 15 mg/L, 15 ml 15 mg/L, 15 ml 15 mg/L, 15 ml 10 mg/L, 100 ml 20 mg/L, 100 ml 30 mg/L, 100 ml 40 mg/L, 100 ml
300 Watt Xenon Lamp 5 × 8 Watt Blacklight Fluorescent Lamps Sunlight Sunlight 500 Watt Xenon Lamp 500 Watt Xenon Lamp 500 Watt Xenon Lamp 100 Watt Tungsten Lamp 100 Watt Tungsten Lamp 100 Watt Tungsten Lamp 100 Watt Tungsten Lamp
90 min 180 min 180 min 180 min 180 min 30 min 40 min 75 min 90 min
90% 90% 90% 90% 90% 100% 100% 100% 85%
[42] [43] [44]
La2O3/SiO2@SeO2-MCM-41 Core-shell
This This This This
study study study study
shows a comparison between the results of photocatalytic degradation of DCP in deionized water and in three real samples. It clearly shows the potential of the prepared photocatalyst for real samples. Slightly lower efficiency of La–SiO2@SeO2-MCM-41 in real samples might be because of various unknown organic compounds or micro-organisms that might occupy active catalytic sites of the photocatalyst. Different initial characteristics of aquatic media in terms of pH, turbidity, dissolved solids and etc. also can affect the efficiency of the photocatalyst. Apparently high turbidity and TDS are the most effective parameters on decreasing the final efficiency. Fig. S7 shows changes of UV spectra and concentration of DCP vs irradiation time for aforementioned photocatalytic experiments in real samples.
related to the electric charge of DCP itself and the hydrolysis of the surface of photocatalyst in various pH values. Also alkaline pH resulted in very noticeable changes in spectral behavior of DCP.
3.4. Recyclability of La–SiO2@SeO2-MCM-41
3.5.3. Effect of initial concentration of DCP 100 ml of various concentrations of DCP were tested in presence of 30 mg of La–SiO2@SeO2-MCM-41. The results are shown in Table 3. All concentrations less than 30 mg/L of DCP were completely degraded in less than 75 min.
3.5.2. Effect of loaded amounts of photocatalyst Various amounts of La–SiO2@SeO2-MCM-41 from 10 to 100 mg were added to 100 ml of DCP (40 mg/L) and the dosage effect of photocatalyst was investigated. Low amounts of photocatalyst resulted in less photodegradation and also dosage more than 30 mg/100 ml of DCP (40 mg/L) showed less efficiency that can be because of shadowing effect of catalyst powder that prevent the light from penetrating the solution. All results are shown in Fig. S10.
The recyclability of the catalyst is one of the important parameters in heterogeneous processes. In order to investigate the recyclability of our photocatalyst, subsequent to each photodegradation experiment, La–SiO2@SeO2-MCM-41 was separated by centrifuge, washed with hot distilled water, sonicated in ethanol and dried at 80 °C for 2 h. Then, the recovered catalyst was weighted by an analytical balance and used for the next photodegradation experiment. This process was repeated for at least 5 times. After 5 cycle of recovery, the total efficiency in photodegradation of DCP dropped less than 50% compared to fresh substrate (Fig. S8).
3.5.4. Effect of nominal power of light source In current work the conventional tungsten lamps from a local company (Pars Shahab Lamp Co.) were used as light sources. The same type of lamp with different nominal power was purchased (Fig. S11). The lamp with 40 Watt of nominal power was almost ineffective in photodegradation of DCP. By increasing the power of lamps the total photocatalytic efficiency increased but 200 Watt lamp was just slightly better than the conventional 100 Watt lamp plus the 200 Watt light source produced a lot of heat that the ventilated chamber of reaction was not capable of exhausting it very well. This experiment showed that to observe a distinguished photocatalytic reaction at least a nominal 60 Watt tungsten lamp is needed. In order to determine the extent of decomposition of DCP, total organic carbon (TOC) were measured after each experiment (lables of each point in Fig. S11.) Increasing the power of lamp actually decreased the amount of TOC that can be assigned to decomposition of DCP. Ratio of C/C0 is also proportional to TOC values
3.5. Operational parameters and their effect on photocatalytic efficiency In order to investigate the influence of pH, loading of photocatalyst (dosage), initial concentration of DCP, nominal power of light source and salt effect (ionic strength of solution) on photocatalytic activity of La–SiO2@SeO2-MCM-41 some experiments were designed. 3.5.1. Effect of pH The best operational pH was found to be 5.5 and lower or higher pH values diminished the photocatalytic efficiency (Fig. S9). This can be 7
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Fig. 8. Antibacterial activity of prepared substrates against different pathogenic bacteria.
which prove that UV spectra can be used as a reliable tool to determine the extent of decomposition.
3.8. Comparison with other catalysts Table 3 summarizes details of some recent studies using SeO2 or Se containing supports and compares the results with those of the current study. As shown in Table 3, the present study shows some advantages compared to the previously reported works. These include: (1) the photocatalyst showed good efficiency for higher sample volume and concentration with a consumption of low dose of photocatalyst (0.03 g in 100 ml solution), (2) higher efficiency for higher concentrations of DCP (100% DCP degradation for 30 mg/l of DCP within 75 min), (3) The shorter time for nearly complete degradation, (4) use of low-energy tungsten lamp as source of irradiation.
3.6. Salt effect Photocatalytic experiment was executed in presence of various concentrations of NaCl as a typical species present in many real water media. Salts can alter the ionic strength of solution and changes the behavior of both targeted pollutant and catalyst. Results are shown in Fig. S12. By increasing the concentration of NaCl the efficiency of photodegradion were declined. Rajabi et al. [35] mentioned some reasons for this phenomenon that are related to the presence of Cl− anions; first, competition between Cl− and pollutant (here DCP) to reach the surface of the catalyst. Second, Cl− can act as an h+ trap and prevent it from engaging in formation of reactive radicals.
4. Conclusions La–SiO2@SeO2-MCM-41 core-shells were synthesized via simple solgel method. Distribution of SeO2 into a mesoporous matrix resulted in a considerable photocatalytic activity. Introduction of La3+ to the coreshell improved the photocatalytic efficiency which may be assigned to electron-scavenging effect of La3+. Our comprehensive experimental study proved that presence of both SeO2 and La3+ lead to achieve a highly efficient photocatalyst based on SiO2 core and MCM- 41 shell. Prepared core-shell catalyst showed a reasonable ability in degradation of DCP in real aquatic samples. Also its antibacterial activity was noticeable.
3.7. Antibacterial activity The antibacterial activity of La–SiO2@SeO2-MCM-41 micro-particles against pathogenic bacteria (including gram-positive streptococcus iniae and gram-negative aeromonas hydrophila) were investigated through measuring inhabitation zone diameter (Fig. 8). It is clear that these substrates show certain level of antimicrobial activity against the investigated pathogens. Zone of inhibition (ZOI) for La-containing substrates were wider than the ones without La. In a previous study [36], it was suggested that the penetration of reactive oxygen species (ROS), probably produced on the surface of photocatalysts, onto the cell membrane can cause to damage of cellular proteins and cell death. Furthermore, the obtained results indicated that the streptococcus iniae is the most susceptible bacteria with almost ZOI of 18 mm when was exposed to 20 mg/l concentration of La–SiO2@SeO2-MCM-41 dispersed in dimethyl sulfoxide. Aeromonas hydrophila showed some resistance and ZOI was about 12 mm. The highest microbial resistance to La–SiO2@SeO2-MCM-41 was seen for Staphylococcus aureus. Bacterial cells are surrounded by a complex and multilayer wall which act as their defense against many antibacterial agents. The cell wall of bacteria consists of a thin or thick layer of peptidoglycan, surrounded by a lipopolysaccharide (LPS) membrane which is the main barrier to protect them [36–38]. Any form of weakness in this walls can end up in penetration of reactive agents (here reactive oxygen species) and consequent damages. Higher susceptibility of gram–negative bacteria to metal oxide NPs has been reported for CuO NPs, previously [39].
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