Multimorphology Mesoporous Silica Nanoparticles for Dye Adsorption

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 3533−3545

pubs.acs.org/journal/ascecg

Multimorphology Mesoporous Silica Nanoparticles for Dye Adsorption and Multicolor Luminescence Applications Jie Chen,† Ye Sheng,† Yanhua Song,† Meiqi Chang,† Xiangting Zhang,† Lei Cui,† Deyue Meng,† He Zhu,† Zhan Shi,‡ and Haifeng Zou*,† †

College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China

‡

ABSTRACT: A series of mesoporous SiO2 nanoparticles (MSNs) have been synthesized by a modified sol−gel process. The morphology could be manipulated from flower-like nanospheres to flower-like nanodisks, circular nanodisks and sunken nanovesicles by simply adjusting the amount of ammonia. Among the various morphologies, the flower-like nanospheres (N1) with a large specific surface area of 1037.16 m2 g−1 exhibited the highest and fastest adsorption capacity for dye-RhB because of the most complex surface structure (234.61 mg g−1, approximately 90% in the first 5 min). The RhB adsorption processes on different MSNs were explained by Langmuir, Freundlich and Temkin isotherm models, and a transition from multilayer adsorption to homogeneous adsorption was achieved. The adsorption kinetics was in accord with the pseudo-second-order model, suggesting the rate-limiting step of RhB on MSNs was mainly chemical process. The luminescence properties of different samples revealed that the nanovesicles structure (N5) exhibited the strongest luminescent intensity due to the least surface defects. By reasonably adjusting the doping concentration ratio of Eu3+ and Tb3+ ions, multicolor emissions of red, orange-red, orange, yellow and green can be easily achieved. The excellent adsorption and luminescence performances indicate that the as-prepared multimorphology MSNs have promising applications on efficient wastewater treatment and multicolor optical devices. KEYWORDS: Mesoporous silica nanoparticles, Multimorphology, Color-tunable, Adsorption, Luminescence

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INTRODUCTION Synthetic dyes as common colorants have been widely used in textiles, plastic, paper, cosmetics, leather, printing and other industries.1,2 However, dye molecules and their metabolites are toxic and difficult to biodegrade. Once the wastewater containing dye is discharged into the environment, it will not only destroy the balance of the water environment but also damage the vital organs such as brain, kidney, liver and reproductive system, which poses a serious threat to human health and ecological system.3,4 Therefore, numerous methods have been employed to remove dye from industrial wastewater, including chemical coagulation, photodegradation, trickling filters, membrane separation and adsorption.5−7 Among the various methods, adsorption technique is considered as the most convenient and promising method for treating largevolume dye wastewater because of its low cost, high efficiency and simple operation.8 Recently, mesoporous SiO2 materials have attracted considerable interest as adsorbents for the removal of various dyes from wastewater due to their highly porous, ease to synthesize and excellent chemical stability.9 For example, Dong et al. reported that mesoporous SBA-15 exhibited excellent adsorption performance for methylene blue.10 T. Jesionowski reported © 2018 American Chemical Society

that amino-functionalized spherical silica showed a good adsorption capacity on Blue 19.11 Huang et al. reported that long fibrous mesoporous SBA-15 could effectively adsorb different cationic dyes from aqueous solutions.12 However, the reported morphologies of mesoporous silica materials are mainly zero/one-dimensional structures, whereas the complex 3D nano/microstructures are rare.13 It is well-known that morphology and surface characteristics have serious influence on adsorption performance. The samples with complex surface structure usually exhibit superior dye adsorption capacity due to their large surface area can provide abundant interactive sites for organic pollutants.14 Hence, exploring a facile method for synthesizing unique mesoporous SiO2 materials with different morphologies to investigate the relationship between the surface characteristic and dye adsorption capacity is of great significance. In addition, SiO2 is a promising substrate for preparing luminescent materials due to its high optical transparency. And as a member of IV oxide, SiO2 can be easily synthesized by sol− Received: October 24, 2017 Revised: December 28, 2017 Published: January 29, 2018 3533

DOI: 10.1021/acssuschemeng.7b03849 ACS Sustainable Chem. Eng. 2018, 6, 3533−3545

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. SEM images and size distributions of SiO2 nanoparticles (N1−N5) obtained with different amount of NH3·H2O (a) 0.1 mL, (b) 0.2 mL, (c) 0.3 mL, (d) 0.4 mL, (e) 0.5 mL.

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gel method at low temperature.15 Hence, if phosphors are prepared in this way, one can effectively avoid the harsh experimental conditions of traditional methods, such as prolonged high temperature or high pressure required for solid phase and hydrothermal methods.16,17 Moreover, the network structure of SiO2 has a good protective effect on the lanthanide ions doped inside.18 And there is rare report about the relationship between different surface structure and luminescence property. Therefore, the purpose of this study is to synthesize a series of mesoporous silica nanoparticles (MSNs) with larger surface area by simple sol−gel method, and explore their adsorption and luminescence applications. In this work, the complicated surface structure and the abundant hydroxyl groups in the MSNs can provide abundant active sites for organic pollutants and lanthanide ions. The adsorption behaviors of different MSNs were evaluated by selecting RhB as the model dye, and the luminescence properties were investigated by doping Eu3+ and Tb3+ ions. The relationships between structures and adsorption and optical performances were investigated in detail in this work.

EXPERIMENTAL SECTION

Materials. Pentanol, absolute ethanol, ammonia (NH3·H2O, 28%), cetyltrimethylammonium bromide (CTAB) and rhodamine B (RhB) were bought from Beijing Chemical Reagents. Tetraethoxysilane (TEOS) was obtained from Aladdin Reagent. All chemical reagents were analytical grade and used without further purification. Eu(NO3)3 and Tb(NO3)3 was obtained by dissolving Eu2O3 (99.99%) and Tb4O7 (99.99%) in dilute HNO3 solution by continuously stirring at high temperature. Synthesis of Multiple Morphologies MSNs. The different morphologies of MSNs were synthesized by using CTAB as surfactant and porogen, whereas pentanol as the cosolvent. In a typical synthesis procedure, 0.35 g of CTAB was disolved into 5 mL of ethanol and 20 mL of H2O with stirring to form a transparent solution, and 0.5 mL of pentanol was dropped into the mixture to obtain a microemulsion environment. Then different amounts of NH3·H2O (0.1, 0.2, 0.3, 0.4 and 0.5 mL) and 2.2 mL of TEOS were added. After additional stirring for 6 h, the white precipitate was separated and washed with ethnol and water for several times, then dried at 60 °C in an oven for 12 h. Finally, CTAB and other organic components were removed by calcination the product in air at 600 °C for 2 h. 3534


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Figure 2. SEM images of sample N1 for different reaction time (a) 20 min, (b) 40 min, (c) 1 h, (d) 2 h, (e) 6 h.

Figure 3. Illustration of the synthetic procedure of MSNs with different morphologies Synthesis of Eu3+, Tb3+ Doped Multiple Morphologies MSNs. The synthesis process of the Eu3+, Tb3+ doped multiple morphologies MSNs is similar to the steps above, except for the different amounts of Eu(NO3)3 and Tb(NO3)3 (mole ratio of Eu/Si = 0, 0.005, 0.010, 0.015, 0.020, 0.025, 0.030, 0.040; Tb/Si = 0.040) added into the mixture after the addition of TEOS. Adsorption of Rhodamine B. The adsorption performance of the obtained multiple morphologies MSNs was investigated by using RhB dye as the probe. In brief, 20 mg of adsorbent was uniformly dispersed into 20 mL of RhB aqueous solution by stirring for a certain time at room temperature. A range of RhB concentrations from 5 to 360 mg L−1 at PH = 5.8 were used to study the maximum adsorption capacity of MSNs. The RhB adsorption abilities at different stirring times and different pH values were evaluated with an initial concentration of 20 mg L−1. The pH values of the RhB solutions were adjusted by using 0.1 mol L−1 HCl and 0.1 mol L−1 NaOH solutions. The concentration of RhB was calculated by measuring the corresponding absorption peak intensity of RhB at 552 nm using a UV−vis spectrophotometer. The adsorption capacity can be calculated using eqs 1 and 2.19

Qt =

(C0 − Ct )V m

(1)

Qe =

(C0 − Ce)V m

(2)

The RhB removal efficiency was calculated as

Removal (%) =

C0 − Ce × 100% C0

(3)

Where Qt (mg g−1) and Qe (mg g−1) are the adsorption capacities of RhB at t min and equilibrium. C0 (mg L−1) is the initial concentration of RhB, Ct (mg L−1) is the RhB concentration after adsorption at t min, Ce (mg L−1) is the equilibrium concentration of RhB, V (L) is the volume of the RhB solution and m (g) is the mass of MSNs adsorbent. Reusability of MSNs. After RhB adsorption, SiO2-RhB samples were collected and washed with distilled water for removing the unabsorbed dyes. Then SiO2-RhB samples were added into alcohol for stirring 30 min, and then thoroughly washed with distilled water. Finally, the regenerated SiO2 samples were dried and reused in the next cycle of dye adsorption experiments. Characterization. The structure of the product was examined by XRD on a Rigaku D/max-B X-ray diffractometer with Cu Kα radiation, and Fourier transform infrared spectra (FT-IR) on a PerkinElmer 580B infrared spectrophotometer. The morphology was observed by scanning electron microscopy (SEM) equipped with H JEOL JXA-840 EDX system (S-4800, Hitachi). The Brunauer−Emmett−Teller (BET) analysis was performed using a specific surface area analyzer (Micromeritics, ASAP2020HD88) and the pore size distribution was analyzed by BJH method. The UV−vis spectra were measured using a UV−vis spectrometer (UV-2450). The fluorescence spectrum were 3535


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Figure 4. XRD pattern of sample N1−N5 (a), EDS spectrum of sample N1-calcined (b), FT-IR spectra of sample N1−N5 (c), and N1-uncalcined, N1-calcined, N1-RhB (d).

extended to 6 h, the uniform monodisperse flower-like SiO2 nanospheres with vast wrinkles on the surface are obtained. Based on the statements discussed, the possible formation mechanism of MSNs with different morphologies (N1−N5) is speculated according to a self-assemble process20 as shown in Figure 3. First, pentanol as a cosolvent is added into the aqueous solution to constitute an oil-in-water microemulsion environment. Simultaneously, CTAB as a surfactant aggregates and assembles into flower-like micelle molecules in the solution. Second, TEOS rapidly diffuses into oil phase once added into the solution, and the negatively charged silicate molecules hydrolyzed by TEOS are arranged at the space among the CTAB micelle molecules. Here, NH3·H2O is the catalyst to control the hydrolysis and condensation rate of TEOS, which seriously affects the morphology of the final product. When the amount of NH3·H2O is 0.1 mL, the hydrolysis and condensation reactions are slow, the anisotropic growth is dominant and SiO2 nanospheres are obtained. The wrinkles on the surface of nanospheres are due to some volatile components in the unstable emulsion constantly escaping from the condensed silica shell.21 As the NH3·H2O content increases from 0.2 to 0.5 mL, the TEOS hydrolysis rate gradually accelerate and more silicate molecules aggregate around CTAB micelles, resulting in an asymmetrical growth and a relatively thick SiO2 shell. Thus, the samples gradually transform from flower-like nanopheres to flower-like nanodisks, to circular nanodisks, and to sunken nanovesicles. Finally, all the silicates molecules are completely condensed into SiO2 and a series of multiple morphologies SiO2 nanoparticles are obtained. The structure and composition of MSNs with different morphologies (N1−N5) are characterized by XRD, EDS and

measured using a luminescence spectrophotometer with 150 W xenon lamp as excitation source (Horiba Jobin Yvon, JYFM-4).

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RESULTS AND DISCUSSION Morphology, Structure and Surface Properties of MSNs. Figure 1 presents the SEM images and size distributions of MSNs with different morphologies (N1−N5). It can be easily seen that the ammonia content seriously affects the size and morphology of the obtained product. As shown in Figure 1a, the sample using 0.1 mL of NH3·H2O (N1) is composed of unique flower-like nanospheres with many small wrinkles on the surface and the average diameters are about 157 nm. When the amount of ammonia is increased to 0.2 mL (N2), many concavities are occurred at the center of the flower-like nanospheres, which makes the samples look like flower-like nanodisks, and the diameters are approximately 201 nm. As the amount of ammonia is adjusted to 0.3 mL (N3), the wrinkles on the surface of nanoparticles tail off, while the diameters increase to 216 nm. With the amount of ammonia increases to 0.4 mL (N4), uniform discoidal nanoparticles with the diameter about 235 nm are formed. Continuing increasing the amount of ammonia to 0.5 mL (N5), many sunken nanovesicles are obtained with the diameter about 317 nm. In order to understand the morphological evolution process clearly, we take the sample N1 as an example and a series of samples are prepared at different reaction time. As shown in Figure 2a−e, a lot of spherical nanoparticles are formed when the reaction time is 20 min. With the extension of reaction time, the surface of the spherical nanoparticles becomes wrinkled due to the continuous hydrolysis and condensation reactions of TEOS molecules. When the reaction time is further 3536


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Figure 5. N2 absorption−desorption isotherms and the corresponding pore size distributions of MSNs with different morphologies (a) N1, (b) N2, (c) N3, (d) N4 and (e) N5.

C−H (1478 cm−1) vibrations24 disappear in the N1-calcined sample, indicating the surfactant CTAB is eliminated completely after calcination at 600 °C.25 By comparing N1calcined sample, some small peaks attributed to the carbonyl, benzene ring and C−N group vibrations occur at 1598, 1412 and 1346 cm−126 in N1-RhB sample, implying RhB molecules have been successfully adsorbed on the MSNs. Moreover, compared with N1-calcined sample, the peak around 3391 cm−1 decreases obviously, indicating some weak interactions might exist between RhB molecules and the surface hydroxyl groups on the MSNs. N2 Adsorption−Desorption Analysis. Figure 5 shows the N2 adsorption−desorption isotherms of MSNs with different morphologies. Inset: Pore size distribution curves of the asprepared samples. For sample N1 to N5, the particle sizes

FT-IR. As shown in Figure 4a, all samples exhibit a similar broad diffraction peak at 2θ = 23°, which can be matched well to the amorphous SiO2. The EDS spectrum (Figure 4b) of sample N1 shows that only Si and O elements are present in the calcined samples. As shown in Figure 4c, the FT-IR spectra of five samples are similar. The sharp peaks at 1082, 801 and 465 cm−1 are assigned to the asymmetric and symmetric stretching vibration, and bending vibration of Si−O−Si and Si− O bonds.22 The broad absorption bands at 3391 cm−1 and the weak peaks at 1638 cm−1 are attributed to −OH asymmetric stretching and bending vibrations.23 Additionally, some weak peaks associated with Si−OH vibration can also be observed at 1518 and 951 cm−1. By comparing the FT-IR spectra of N1 sample before and after calcination, it can be found that the peaks assigned to −CH3 (2927 cm−1), −CH2 (2854 cm−1) and 3537


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the adsorbed amounts of RhB are 19.78, 19.65, 19.62, 19.53 and 19.33 mg g−1, corresponding to the removal rates of 98.92%, 98.25%, 98.08%, 97.65% and 96.65%. This indicates that the equilibrium adsorption of RhB on the as-prepared samples can be easily reached within a short time. In addition, as listed in Table 2, the RhB adsorption capacity on the asprepared MSNs is much higher and faster than that of other adsorbents reported in the literature.

gradually increase from 157 to 201, 216, 235, 317 nm, while the specific surface area (SBET) reduce from 1037.16 to 994.28, 715.27, 640.53 and 515.76 m2 g−1 due to more and more smooth surface. According to IUPAC classification, all five samples exhibit typical type IV isotherms with obvious hysteresis loops at relative pressure (P P0−1) of 0.4 to 1.0, implying the existence of mesoporous structure.27 Meanwhile, N1 and N2 exhibit H-3 hysteresis loops, indicating the presence of slit-shaped pores due to the aggregates of plate-like wrinkles on the surface,28 which is consistent with the SEM images of N1 and N2 samples. Meanwhile, sharp peaks centered at 2.01 and 2.14 nm can be observed on the pore size distributions curves of N1 and N2. As shown in Figure 5c−e, the isotherms of N3, N4 and N5 samples show obvious H-2 hysteresis loops, meaning a broad distribution of pore size and shape.29 The pore size distribution curves also demonstrate that a series of irregular pores exist in the N3, N4 and N5 and the average diameters are 3.7, 4.0 and 4.7 nm (calculated from the adsorption branches). Table 1 summarizes the Size, SBET and

Table 2. Comparison of the Maximum Adsorption Capacity of Different Adsorbents

Tea Waste [Ni(bipy)2]2(HPW12O40) Graphene oxide/silicalite-3 composites SnS2/rGO composite Waste of seeds Single-wall carbon nanotube film MSN-N1

Table 1. Size, SBET and Vtotal of MSNs with Different Morphologies Samples

V(NH3·H2O) (mL)

N1 N2 N3 N4 N5

0.1 0.2 0.3 0.4 0.5

2

−1

3

SBET (m g )

Vtotal (m g )

± ± ± ± ±

1037.16 994.28 715.27 640.53 515.76

1.18 1.15 0.64 0.57 0.51

12 14 12 14 22

Qmax (mg g−1)

1.0 g L−1, 2 h 1.0 g L−1, 40 min 1.0 g L−1, 1 h

31.94 22.75 56.55

26 30 31

2.0 g L−1, 30 min 2.5 g L−1, 40 min 1.0 g L−1, 4 h

94.07 117.0 190.0

32 33 34

1.0 g L−1, 40 min

234.61

This work

−1

Size (nm) 157 201 216 235 317

Experimental conditions

Adsorbent type

Reference

Adsorption Isotherms. It is well-known that adsorption isotherm is an important parameter to comprehend the interaction between adsorbent and adsorbate. Thus, Langmuir, Freundlich and Temkin isotherm models35 are fitted in our work to analysis the adsorption process of RhB on the asprepared MSNs. The Langmuir isotherm model assumes a homogeneous adsorption surface, which is given by the following equation:

Vtotal of five samples (N1−N5). It is worth noting that all samples possess relatively high SBET and Vtotal, indicating that they are promising matrix materials for adsorption, storage and bonding many other organic molecules such as dye, drug, etc. Factors Affecting RhB Adsorption on MSNs. In order to assess the dye adsorption performance, different morphologies SiO2 nanoparticles were immersed into RhB aqueous solution. The effects of pH value, initial concentration and contact time of RhB adsorption on all five samples are investigated. As shown in Figure 6a, as the pH changes from 3 to 11, the adsorptions (Qe) increase slightly at first, reaching a maximum at pH 5, and then drop quickly due to the competitive adsorption of the hydroxyl ions in the solution. In Figure 6b, with the initial concentration increase from 5 to 360 mg L−1, the equilibrium adsorptions (Qe) gradually increase, and the maximum adsorptions for N1 to N5 are 234.61, 233.39, 217.94, 174.33 and 166.39 mg g−1, respectively. In Figure 6c, the removal rates of RhB for five samples are approximately 90% in the initial five minutes, implying a strong interaction of RhB on MSNs samples. After 40 min, the adsorption is equilibrium and

Qe =

KLQ maxCe 1 + KLCe

(4)

The Freundlich isotherm model is applicable to a multilayer adsorption surface, which is presented as

Q e = KFCe1/ n

(5)

The Temkin isotherm model is an empirical model for studying the heat of adsorption, which is given as Q e = B ln KT + B ln Ce

(6)

Where Qmax (mg g−1) is the RhB maximum adsorption capacity, KL and KF are the adsorption constants, KT (L g−1) is the equilibrium binding constant and B is Temkin constant. According to each isotherm models, the fitted adsorption isotherms of RhB on different MSNs are plotted in Figure 7, and the related parameters are calculated and given in Table 3.

Figure 6. Effects of (a) pH value, (b) initial concentration and (c) contact time of RhB adsorption on different samples (conditions: dosage,1.0 g L−1; initial concentration of RhB solution, 20 mg L−1; initial pH value, 5.8). 3538


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Figure 7. Langmuir (a), Freundlich (b), Temkin (c) models and Langmuir parameter (RL) (d) for RhB adsorption on different samples.

Table 3. Langmuir, Freundlich and Temkin Adsorption Isotherm Parameters for RhB Adsorption on Different Samples Isotherm models

Parameters

N1

N2

N3

N4

N5

Langmuir

R2 Qmax (mg g−1) KL (L mg−1) R2 KF (L g−1) N R2 B (J mol−1) KT (L mg−1)

0.9754 245.700 0.1756 0.9807 50.528 2.862 0.8159 25.805 31.369

0.9734 243.309 0.1329 0.9792 40.592 2.509 0.8218 29.370 11.717

0.9825 228.833 0.1137 0.9873 36.51 2.561 0.8104 22.792 30.141

0.9979 182.149 0.1272 0.9338 20.949 2.082 0.9677 26.890 4.300

0.9948 176.678 0.0754 0.8969 15.530 1.941 0.9809 27.638 2.228

Freundlich

Temkin

One can see that the R2 values of the Freundlich model are higher than Langmuir and Temkin models for sample N1 to N3, whereas the Langmuir model achieves the highest R2 values for sample N4 and N5. It means that the multilayer adsorption is dominant for N1 to N3, whereas a homogeneous adsorption is occurred for N4 and N5 because of more and more smooth surface. In the Temkin isotherm model, the R2 values for N4 and N5 are higher than N1 to N3, indicating the adsorption processes of N4 and N5 are partial to the uniform distribution of binding energy, which is agreement with the result of homogeneous adsorption. Based on the results of Langmuir model simulation, the maximum adsorption capacities of RhB for sample N1 to N5 are 245.7, 243.309, 228.833, 182.149 and 176.678 mg g−1, which are close to the experimental data. In addition, the values of n in Freundlich model are all higher than 1, indicating a good affinity between the as-prepared samples and dye-RhB in our work. Further, the Langmuir parameter (RL) is another parameter to judge the favorability of adsorption process, which is given as

RL =

1 1 + KLC0

(7)

Generally, the adsorption process is considered irreversible if RL = 0, favorable if 1 > RL > 0, linear if RL = 1.0 and unfavorable if RL > 1.0.36 Figure 7d shows the RL values versus the initial concentration (C0) for different samples. From this figure, it can be seen that the RL values are all between 0 and 0.8 for N1 to N5, indicating the adsorptions of RhB on MSNs are favorable and useful processes. In addition, the RL values decrease gradually with the initial RhB concentration changes from 4 to 360 mg L−1, indicating the adsorption is more favorable at higher concentration due to the large driving force.37 Adsorption Kinetics. The adsorption rate controlling steps and possible adsorption mechanism of RhB on the as-obtained samples are investigated using the pseudo-first-order, pseudosecond-order, Elovich equations and intraparticle diffusion kinetic equations.38 The pseudo-first-order kinetic model is presented as 3539


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Figure 8. Pseudo-first-order (a), pseudo-second-order (b), Elovich (c) and intraparticle diffusion kinetics (d) for the adsorption of RhB on different samples.

Table 4. Kinetic Parameters Calculated from the Pseudo-First-Order Kinetics, the Pseudo-Second-Order Kinetics, Elovich Kinetics and the Intraparticle Diffusion Kinetics Models Kinetics models Pseudo-first-order kinetics

Pseudo-second-order kinetics

Elovich kinetics Intraparticle diffusion kinetics

ln(Q e − Q t ) = lnQ e − K1t

Parameters

N1

N2

N3

N4

N5

R2 K1 (min−1) Qe,cal (mg g−1) R2 h (mg g−1 min−1) K2 (min−1) Qe,cal (mg g−1) R2 b (g mg−1) R2 Ki1 (mg g−1 min−1/2) C1 (mg g−1) R2 Ki2 (mg g−1 min−1/2) C2 (mg g−1) R2 Ki3 (mg g−1 min−1/2) C3 (mg g−1)

0.6313 0.0388 2.4359 0.9999 65.21 0.1632 19.9681 0.9816 2.1142 0.9909 1.8476 16.1374 0.9959 0.5481 17.4539 − 0.2915 17.9363

0.6986 0.0417 3.0884 0.9999 53.53 0.1345 19.9402 0.9209 1.4362 0.7090 4.4193 13.1586 0.9138 0.8405 16.3211 − 0.24293 18.1136

0.7919 0.0567 3.7081 0.9999 54.28 0.1389 19.8059 0.9092 0.9758 0.7579 4.0280 11.5732 0.8432 1.1990 15.2903 − 0.2321 18.1519

0.8039 0.0545 3.8135 0.9999 50.42 0.1294 19.7746 0.9017 0.9784 0.9965 4.5791 11.0778 0.7923 0.7348 16.4734 − 0.2591 17.8912

0.8206 0.0559 4.5299 0.9999 39.19 0.1015 19.7006 0.8994 0.8171 0.4598 4.4895 9.9771 0.9285 0.9139 15.6711 − 0.3509 17.1108

Where K1 (min−1) and K2 (g mg−1 min−1) are constant. And the initial adsorption rate h (mg g−1 min−1) can be calculated by the following equation:

(8)

The pseudo-second-order kinetic model suggests that the adsorption capacity is related to the number of active sites on the surface, which is expressed as t 1 t = + 2 Qt Q K 2Q e e

h = K 2Q e 2

(10)

The Elovich kinetic model is suitable to describe activated chemisorption:

(9) 3540


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ACS Sustainable Chemistry & Engineering Qt =

1 1 ln(ab) + ln t b b

(11)

Where a (mg g−1 min−1) is Elovich constant and b (g mg−1) is a coefficient corresponding to the activation energy of adsorption. The intraparticle diffusion kinetics model is used to explore whether the adsorption is controlled by more than one diffusion behaviors, which is given as Q t = K it 1/2 + C i −1

(12) 1/2

Where Ki (mg g min ) is the intraparticle diffusion rate constant and Ci (mg g−1) is related to the boundary layer thickness. Figure 8 shows the fitting adsorption kinetics models and the corresponding parameters are summarized in Table 4. It can be seen that the R2 values of N1 to N5 in the pseudo-second-order kinetics model more approach 1, indicating a better linearity than the pseudo-first-order kinetics model. Moreover, the equilibrium adsorption values (Qe,cal) calculated by the pseudosecond-order kinetic model are closer to the experimental results. Thus, we speculate that the rate-limiting step of the RhB adsorption on MSNs is mainly chemical process involves electron sharing or exchange,39,40 whereas the physical interaction between adsorbent and pores is relatively weak. In addition, h and K2 values of N1 are higher than those of other samples, indicating the initial adsorption rate of N1 is the fastest. The Elovich kinetics model shows that the R2 values gradually decrease from 0.9816 to 0.8894, as well as b values reduce from 2.1142 to 0.8171 for the RhB adsorption on N1 to N5 samples. That is to say the chemical activation energy of N1 is higher than other samples, which is in agreement with the maximum adsorption capacity.38 The fitting of the intraparticle diffusion model does not through the origin (0, 0), indicating the diffusion is not the only rate-limiting step of the RhB adsorption on MSNs.37 And with the extension of time, the intraparticle diffusion gradually weakened, which is consist with the variation of Ki1, Ki2 and Ki3 values. On the basis of the adsorption kinetics models discussed, we speculate that the adsorption process of RhB on MSNs can be divided into three stages. First, the initial adsorption stage, RhB molecules rapidly spread to the surface of MSNs and a very fast adsorption occurs because of the large number of reaction sites on the surface of MSNs. During the subsequent stage, the RhB adsorption process is largely limited by the intraparticle diffusion effect, so the adsorption rate becomes slower. Finally, in the adsorption equilibrium stage, the intraparticle diffusion rate is very slow because of the concentration dye-RhB is extremely low.40 Thus, it can be deduced that surface adsorption and intraparticle diffusion together affect the adsorption of RhB on MSNs. The main chemical reaction during the surface adsorption process is the strong hydrogen bond interaction between a large number of active hydroxyls on the surface of MSNs and the nitrogen atoms or carbonyl in RhB dye molecules.41 In this study, N1 exhibits the highest RhB adsorption capacity, which is probably because its largest surface area could provide more interactive sites for dye molecules.42 Reusability Study. Considering the economic benefit and feasibility, regeneration potential of MSNs adsorbents was studied. The adsorption−desorption cycles of RhB on sample N1 (Figure 9) shows that the adsorption capacity decreases slightly after five consecutive cycles, and the removal efficiency

Figure 9. Removal efficiency of RhB on sample N1 after repeated five cycles.

is still greater than 90%. This indicates that the as-prepared MSNs sample has a good regeneration and reusability, which is suitable for colored wastewater cleanup with high-efficiency and low-cost. Photoluminescence Properties of MSNs Doped with Eu3+ and Tb3+. The XRD patterns, EDS spectrum, UV−vis absorbance spectra and removal efficiency of RhB for Eu3+ and Tb3+ ions doped MSNs sample are investigated. As shown in Figure 10a, the shape and position of XRD diffraction peak do not change after doping Tb3+ and Eu3+ ions, indicating that no Eu2O3 and Tb4O7 impurities are formed. However, the broad diffraction peak at 2θ = 23° decreases obviously after doping Tb3+ and Eu3+ ions, which may be due to the deformation of Si−O−Si network structure in the silica matrix.43 From the EDS spectrum (Figure 10b), it can be easily seen that Si, O, Eu and Tb are uniformly distributed in the Eu3+, Tb3+ codoped N5 sample. From the UV−vis absorbance spectra and removal efficiency (Figure 10c,d) of dye-RhB adsorption on N5 sample, it can be seen that the removal efficiency of dye-RhB on Eu3+ and Tb3+ ions codoped N5 sample is 85.61%, a little lower than the pure N5 sample of 96.65%. This may be due to the part of surface hydroxyl groups in the MSNs sample are attached to rare earth ions in the form of Si−O−Tb or Si−O−Eu bonds,44 which significantly weakens the interaction force between the samples and dye. However, the removal efficiency (85.61%) is still much higher than many other reported adsorbents,45,46 demonstrating the as-prepared rare earth ions doped MSNs are promising bifunctional materials with excellent absorption and fluorescence properties. The dependence of luminescence properties on the morphology was investigated by measuring the excitation and emission spectra of SiO2:Eu3+ and SiO2:Tb3+ samples with different morphologies. As shown in Figure 11, it is obvious that all five samples show an identical spectral pattern with distinct difference in the intensity. For SiO2:4%Eu3+ samples, the excitation spectra show some sharp peaks from 250 to 550 nm with the strongest peak at 393 nm due to the f-f transitions of Eu3+.44 Upon excitation at 393 nm, SiO2:4%Eu3+ samples show a series of typical transitions of the Eu3+ ions including 5 D0-7F0 (577 nm), 5D0-7F1 (590 nm), 5D0-7F2 (611 nm), 5 D0-7F3 (650 nm) and 5D0-7F4 (701 nm).47,48 The excitation spectra of SiO2:4%Tb3+ are composed of a broad band at 229 nm due to the spin-allowed f-d transition, and some weak peaks at 316, 339, 350, 367 and 376 nm ascribed to the characteristic f-f transitions. It is well-known that the excitation wavelength 3541


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Figure 10. XRD patterns (a), EDS spectrum (b), UV−vis absorbance spectra (c) and removal efficiency (d) of dye-RhB on Eu3+, Tb3+ doped N5 sample

Figure 11. Excitation and emission spectra of SiO2:4%Eu3+ (a, b), SiO2:4%Tb3+ (c, d) samples with different morphologies.

monitored at 376 nm, which are consist of a series of peaks of D4-7FJ (J = 6, 5, 4, 3) transitions at 487, 541, 583 and 620 nm,

near 370 nm is expected to meet the typical commercial chips.49 Thus, the emission spectra of SiO2:4%Tb3+ samples are

5

3542


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Figure 12. Comparison of excitation spectra of SiO2:Tb3+, SiO2:Eu3+ and SiO2:Tb3+, Eu3+ samples (a), the emission spectra of SiO2:4%Tb3+, x%Eu3+ sample (b).

respectively.50 When comparing the luminescence intensity of all five samples, the sample N5 exhibits the strongest emission intensity, whereas the sample N1 presents the weakest emission intensity. This may be due to the large specific surface area usually occupies more defects for nonradiative recombination,51 which could reduce the luminescence intensity. Therefore, the nanovesicles (N5) with the smallest specific surface area (515.76 m2 g−1) present the strongest emission intensity. On the other hand, the small wrinkles on the surface of N1 will cause a high scattering of light and a low brightness.52 As a result, the luminescence intensity of the flower-like nanospheres (N1) is lowest. In order to investigate the energy transfer process and tunable emission color, the Eu3+ and Tb3+ codoped SiO2 samples are synthesized. Figure 12a shows the excitation spectra of SiO2:4%Tb3+, SiO2:4%Eu3+ and SiO2:4%Tb3+, 1% Eu3+ samples. By comparing the three lines, it can be found that the excitation spectrum of SiO2:4%Tb3+, 1%Eu3+ sample contains both Eu3+ and Tb3+ excitation peaks when detected under 611 nm (the emission of Eu3+). The excitation peak at 376 nm is prominent, which can be used to excite the Tb3+, Eu3+ codoping samples. Figure 12b illustrates the emission spectra of SiO2:4%Tb3+, x%Eu3+ (x = 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) samples. Although the concentration of Tb3+ ions is fixed, the emission intensity of Tb3+ monotonously decreases as the Eu3+ concentration changes from x = 0 to 3.0, indicating the presence of energy transfer from Tb3+ ions to Eu3+ ions.53 However, the emission intensity of Eu3+ increases at first to a maximum at x = 2.0, and then gradually decreases because of the concentration quenching of Eu3+ ions.54 The CIE chromaticity diagram and CIE coordinates of Tb3+, Eu3+ single and codoped SiO2 samples are shown in Figure 13 and Table 5. It can be found that the CIE chromaticity coordinates of Tb3+/Eu3+ single doping sample N1 to N5 can vary from (0.2528, 0.3469) to (0.2794, 0.4692) in green range, and from (0.4777, 0.3469) to (0.5229, 0.3497) in red range. For SiO2:4%Tb3+, x%Eu3+ samples, the CIE chromaticity coordinates are (0.3638, 0.2938), (0.4045, 0.3063), (0.4268, 0.3122), (0.4547, 0.3175), (0.4409, 0.2936), (0.4271, 0.2871) corresponding to x = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, respectively, and the emission colors change from yellow to red including orange and orange-red. This indicates that the as-obtained products emerge the advantage of multicolor luminescence by simply adjusting the doping concentrations of different rare earth ions, which have promising applications in solid-state lighting systems and optoelectronic devices fields.

Figure 13. CIE chromaticity diagram and a series of digital photographs of SiO2:Ln3+ (Eu, Tb) phosphors.

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CONCLUSIONS In this paper, we developed a facile route for preparing multimorphology mesoporous silica nanoparticles (MSNs) by simply adjusting the amount of ammonia. The as-prepared materials are used as adsorbents to remove RhB from wastewater and matrixs for multicolor luminescence. The removal rates of RhB on MSNs are approximately 90% in the initial 5 min and the maximum adsorptions of RhB for N1 to N5 are 234.61, 233.39, 217.94, 174.33 and 166.39 mg g−1, exhibiting a faster and higher removal capacity than the reported data. Langmuir, Freundlich and Temkin isotherm models demonstrated that the RhB adsorption on different MSNs were favorable, and there is a transition from multilayer to homogeneous adsorption from N1 to N5 due to more and more smooth surfaces. The adsorption kinetics processes show that surface adsorption and intraparticle diffusion together affect the RhB adsorption on MSNs, and the main chemical reaction during the surface adsorption process is the strong hydrogen bond interaction between surface hydroxyl groups and RhB molecules. The luminescent property of Eu3+/Tb3+ single doped SiO2 samples reveals that nanovesicles (N5) show the strongest luminescent intensity due to the least surface defects. And the emission colors of Eu3+ and Tb3+ codoped 3543


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ACS Sustainable Chemistry & Engineering Table 5. CIE Parameters of SiO2:Ln3+ (Eu, Tb) Phosphors Sample no.

Sample

Excitation (nm)

1

SiO2:4%Tb3+ (N1)

376

2

SiO2:4%Tb3+ (N2)

376

3

SiO2:4%Tb3+ (N3)

376

4

SiO2:4%Tb3+ (N4)

376

5

SiO2:4%Tb3+ (N5)

376

6

SiO2:4%Eu3+ (N1)

393

7

SiO2:4%Eu3+ (N2)

393

8

SiO2:4%Eu3+ (N3)

393

9

SiO2:4%Eu3+ (N4)

393

10

SiO2:4%Eu3+ (N5)

393

11

SiO2:4%Tb3+, 0.5% Eu3+ SiO2:4%Tb3+, 1.0% Eu3+ SiO2:4%Tb3+, 1.5% Eu3+ SiO2:4%Tb3+,2.0% Eu3+ SiO2:4%Tb3+, 2.5% Eu3+ SiO2:4%Tb3+, 3.0% Eu3+

376

12 13 14 15 16

376 376 376 376 376

CIE (x, y) (0.2528, 0.3469) (0.2546, 0.3632) (0.2635, 0.4033) (0.2717, 0.4484) (0.2794, 0.4692) (0.4777, 0.3469) (0.4903, 0.3483) (0.5093, 0.3488) (0.5178, 0.3489) (0.5229, 0.3497) (0.3638, 0.2938) (0.4045, 0.3063) (0.4268, 0.3122) (0.4547, 0.3175) (0.4409, 0.2936) (0.4271, 0.2871)

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Color green green green green green red red red red red yellow orange orangered red red red

SiO2 samples change from green to red, orange-red, orange and yellow, by adjusting the Eu3+ concentration. This work suggests that the as-prepared novel MSNs may serve as promising dual functional materials for environmental pollution cleanup and light-display systems.

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AUTHOR INFORMATION

Corresponding Author

*H. Zou. Tel.: +86-0431-85155275; Fax: +86-0431-85155275; E-mail address: [email protected]. ORCID

Zhan Shi: 0000-0001-9717-1487 Haifeng Zou: 0000-0002-1331-2738 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This present work was supported by the National Natural Science Foundation of China (51272085 and 21671078), Jilin Province Science and Technology Development Plan Item (20170312002ZX), the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2016-06), and the Special Project of Provincial School Construction Plan of Jilin Province (SXGJSF2017-3).

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Multimorphology Mesoporous Silica Nanoparticles for Dye Adsorption

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