nanomaterials Article
Synthesis of Nano-Zinc Oxide Loaded on Mesoporous Silica by Coordination Effect and Its Photocatalytic Degradation Property of Methyl Orange Zhichuan Shen 1,2 , Hongjun Zhou 1,2, *, Huayao Chen 1,2 , Hua Xu 1,2 , Chunhua Feng 3 and Xinhua Zhou 1,2, * 1
2 3
*
School of Chemistry and Chemical Engineering, Zhongkai University of Agriculture and Engineering, Guangzhou 510220, China;
[email protected] (Z.S.);
[email protected] (H.C.);
[email protected] (H.X.) Guangzhou Key Lab for Efficient Use of Agricultural Chemicals, Guangzhou 510220, China School of Environment and Energy, South China University of Technology, Guangzhou 510220, China;
[email protected] Correspondence:
[email protected] (H.Z.);
[email protected] (X.Z.); Tel.: +86-020-8900-3114 (H.Z. & X.Z.); Fax: +86-020-8900-3114 (H.Z. & X.Z.)
Received: 28 March 2018; Accepted: 7 May 2018; Published: 9 May 2018
Abstract: Salicylaldimine-modified mesoporous silica (Sal-MCM-3 and Sal-MCM-9) was prepared through a co-condensation method with different amounts of added salicylaldimine. With the coordination from the salicylaldimine, zinc ions were impregnated on Sal-MCM-3 and Sal-MCM-9. Then, Zn-Sal-MCM-3 and Zn-Sal-MCM-9 were calcined to obtain nano-zinc oxide loaded on mesoporous silica (ZnO-MCM-3 and ZnO-MCM-9). The material structures were systematically studied by Fourier transform infrared spectroscopy (FTIR), N2 adsorption/desorption measurements, X-ray powder diffraction (XRD), zeta potential, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ultraviolet diffused reflectance spectrum (UV-vis DRS), and thermogravimetry (TGA). Methyl orange (MO) was used to investigate the photocatalysis behavior of ZnO-MCM-3 and ZnO-MCM-9. The results confirmed that nano ZnO was loaded in the channels as well as the outside surface of mesoporous silica (MCM-41). The modification of salicylaldimine helped MCM-41 to load more nano ZnO on MCM-41. When the modification amount of salicylaldimine was one-ninth and one-third of the mass of the silicon source, respectively, the load of nano ZnO on ZnO-MCM-9 and ZnO-MCM-3 had atomic concentrations of 1.27 and 2.03, respectively. ZnO loaded on ZnO-MCM-9 had a wurtzite structure, while ZnO loaded on ZnO-MCM-3 was not in the same crystalline group. The blocking effect caused by nano ZnO in the channels reduced the orderliness of MCM-41. The photodegradation of MO can be divided in two processes, which are mainly controlled by the surface areas of ZnO-MCM and the loading amount of nano ZnO, respectively. The pseudo-first-order model was more suitable for the photodegradation process. Keywords: nano ZnO; mesoporous silica; salicylaldimine; coordination; photodegradation
1. Introduction Nano zinc oxide (nano ZnO), which is well known for its highly activity and abundance for numerous organocatalysis [1,2], has previously demonstrated its performance in many applications such as the antibacterial field and catalytic degradation field [3,4]. Especially in the field of photodegradation, nano ZnO has attracted extensive attention owing to its strong quantum confinement effects [5]. This nano semiconductor possesses a wide band gap energy as well as Nanomaterials 2018, 8, 317; doi:10.3390/nano8050317
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a large exciton binding energy of about 60 meV, which allows excitonic emissions even at room temperature [6–9]. Thus, it can harvest ultraviolet radiation and generate electron-hole pairs, which will react with oxygen, further leading to the formation of hydroxyl radicals, superoxide radical anions, and hydroperoxyl radicals [10]. However, unsupported nano ZnO has one major drawback as it usually aggregates during the photocatalytic process, which reduces its surface area and thereby reduces the generation of reactive oxygen for the degradation of reagents [11–13]. In order to enhance its dispersion, loading the nano ZnO on a high surface support such as mesoporous silica can solve this problem [14]. In the past decade, mesoporous silica had been considerably made as a structural basis for nanotechnological applications as it is porous, thermally stable, non-toxic and hydrothermally stable [15–19]. Furthermore, as it has uniform and tunable channels with a pore diameter ranging from 2~50 nm, it can also be used to control the size of nanoparticles and make them homodisperse during their formation [6]. A good dispersion can help nanoparticles such as ZnO and TiO2 reduce the light scattering so that they demonstrate better photocatalytic properties [20,21]. These semiconductor nanoparticles can be incorporated inside the mesoporous silica by using various synthesis methods [22]. Among them, the impregnation method is most commonly used to load nano ZnO on mesoporous silica, which can help the nano ZnO form better crystallinity and provide a more direct path for the transfer of electrons, thus increasing photodegradation efficiency [23,24]. However, the limited loading amount of pure mesoporous silica is due to the lack of suitable active adsorption sites, which is still a problem during impregnation [25]. To solve this problem, chemical modification should be introduced to the mesoporous silica. The surface of mesoporous silica is full of silanol groups which makes it easy to graft by chemical groups. Functional group anchoring can be achieved by using a functionalized silane or a co-condensation of silane with the silica or a post grafting of silica [26]. The introduction of organic compounds offers mesoporous silica specific adsorption sites for combination with metal ions. Chouyyok [27] reported that the chelating diamines modification made the mesoporous silica enhance the copper ions adsorption capacity in the waste water application due to the π back-bonding between copper ions and Phen aromatic ring system. He [28] found that the GLYMO-IDA grafted mesoporous silica had a good capacity in the adsorption of nickel ions through its strong binding ability. Such functional modifications can build the chemical bond between mesoporous silica and metal ions with a vacant electron orbit to improve the loading amount of metal ions. Nevertheless, fewer studies have paid much attention to this method in the application of loading metal oxide onto mesoporous silica. Only two studies have reported that nano ZnO were loaded inside the mesoporous silica by functionalizing the surface of MCM-41 using ethylene diamine [29,30]. In this system, the loading amount of ZnO was more than that of using pure MCM-41. Hence, the method of using functionalized mesoporous silica to make more nano ZnO loaded on mesoporous silica requires more research. In this work, salicylaldimine, a kind of Schiff base, was used to adsorb zinc ions as a precursor for ZnO because of their straightforward synthesis and stability as well as their strong coordination with metal ions. On the one hand, owing to the π-system consisting of a benzene ring, phenolic hydroxyl, and carbon-nitrogen double bond, the salicylaldimine can play a key role as a chelating ring [31]. On the other hand, because empty orbitals exist, zinc ions can be regarded as the electron acceptor. These two factors together form the coordination bond between the zinc ion and salicylaldimine. Vasile [32] used a similar Schiff base modified mesoporous silica to adsorb copper ions where the adsorption capacity was 0.92 mmol/g. Its molar ratio of Cu/N was 0.42, which was very close to the theoretical value of 0.5. Salicylaldimine can be synthesized through the reaction of 3-Aminopropyltriethoxysilane with salicylaldehyde [33,34]. Then, by using the synthetic method of co-condensation or post-synthesis, the ethyoxyl of salicylaldimine will react with surface silanol groups of mesoporous silica [35], so that a certain amount of salicylaldimine is grafted onto the mesoporous silica. According to our knowledge, using a Schiff base modified mesoporous silica to adsorb zinc ions as a precursor of nano ZnO has not yet been reported and this method is worth exploring.
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Herein, nano ZnO was loaded onto the surface of MCM-41 by oxidizing the zinc ions caught by the salicylaldimine modified MCM-41. A series of characterization tests were undertaken to analyze the structure of the materials. The properties and the kinetic behavior of the system were also evaluated by photodegradating the methyl orange, a commonly used coloring agent pollutant [36]. 2. Experimental 2.1. Chemicals Cetyl trimethyl ammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), 3-Aminopropyltriethyloxy silane (APTES), and salicylaldehyde were all obtained from Shanghai Aladdin Biochemical Reagents (Shanghai, China). Ethanol, dichloromethane, anhydrous magnesium sulfate, ammonia solution (wt = 25%), zinc nitrate, and methyl orange (MO) were obtained from Tianjin Damao Chemical Reagents (Tianjin, China). All chemicals were analytical grade and used as received without any further purification. 2.2. Synthesis 2.2.1. Preparation of Salicylaldimine According to the literature [34], 8.85 g (39.98 mmoL) of APTES, 4.88 g (39.96 mmoL) of salicylaldehyde, and 100 mL of ethanol were added to a flask and reacted at 95 ◦ C for 3 h. After removing ethanol with rotary evaporation, the intermediate products were dissolved in 10 mL (156.01 mmoL) of dichloromethane. Then, the product was washed with deionized water three times. After being left to stand for 12 h, the organic layer was extracted, and 0.5 g (4.15 mmoL) anhydrous magnesium sulfate was used to remove the residual water. Finally, the final product was filtered to remove dichloromethane and magnesium sulfate to obtain salicylaldimine. According to FTIR (Figure 1), Peaks appearing at 780 cm−1 and 1630 cm−1 of Sal were attributed to the the vibration band of the benzene ring and the stretching band of C=N, which mean that the successful synthesis of salicylaldimine. Nanomaterials 2018, 8, x FOR PEER REVIEW 6 of 21
MCM-41
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1485 1237 1630
2850 2929 ZnO-MCM
1080 480
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Wavenumber (cm-1)
1000
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Figure 1. 1. Fourier-transform spectroscopy (FTIR) spectra of MCM-41, Sal, Sal, Sal-MCM, and Figure Fourier-transforminfrared infrared spectroscopy (FTIR) spectra of MCM-41, Sal-MCM, ZnO-MCM. and ZnO-MCM.
As shown in Figure 2a, the N2 adsorption/desorption isotherms of MCM-41, Sal-MCM-9, and 2.2.2. Preparation of Salicylaldimine-Modified Mesoporous Silica (Sal-MCM) Sal-MCM-3 belonged to the Langmuir IV type adsorption. Hysteresis loops in these isotherms were As reported previously synthesis method prepare not obvious, which meant [37], that the co-condensation samples possessed a small pore was size used [40], to while the Sal-MCM. pore size In a typical process, a total of 1.5 g (4.12 mmoL) of CTAB, 105 mL of ammonia solution (wt 25%), distribution analyzed by the NLDFT method also confirmed the occurrence of micropores, as=shown ◦ and 150 mL deionized water added to the flaskoftoSal-MCM be completely dissolved at 60 C with in Figure 2b.ofThe adsorption andwere desorption branches could not be easily duplicated 1within h of stirring. g (35.28 ofextent TEOSof was added adsorption to the solution dropwise. After 1 h, p/po = 0.0Then, to p/po7.5 = 0.2 due tommoL) a certain chemical by salicylaldimine during 2.5 g of the as-synthesized salicylaldimine was added, and the reaction was stirred for another 6h the testing process [41]. In addition, as shown in Table 1, the modification of salicylaldimine would significantly decrease the BET surface, pore diameter, and pore volume due to the blocking effect of salicylaldimine. Moreover, the BET surface of Sal-MCM-3 and Sal-MCM-9 were 453.385 m2/g and 634.437 m2/g, respectively, which meant that the salicylaldimine modification on Sal-MCM-3 was more than that of Sal-MCM-9. The modification of salicylaldimine not only decreased the pore size of the mesoporous silica, but also broadened the distribution.
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before being crystallized for 24 h at 30 ◦ C, filtered, washed, and dried at 80 ◦ C. At last, the template was removed with 100 mL ethanol at 110 ◦ C for 72 h by the mean of reflux extraction to obtain Sal-MCM-41. Two kinds of samples were prepared and denoted as Sal-MCM-3 and Sal-MCM-9, which represented the modification amount of salicylaldimine as one-third and one-ninth of the mass of TEOS, respectively. Additionally, mesoporous silica without salicylaldimine modification was synthesized and noted as MCM-41. 2.2.3. Preparation of Zinc Oxide Supported by Mesoporous Silica (ZnO-MCM) Zinc oxide nanoparticles were loaded onto mesoporous silica through a wet impregnation method (Scheme 1 A total of 50 mL (100 mmoL) of zinc nitrate solution was added to 500 mg of Sal-MCM-41 or MCM-41 at 30 ◦ C with stirring for 24 h. Then, Zn/Sal-MCM-41 and Zn/MCM-41 was obtained after Nanomaterials 2018, 8, x FOR PEER REVIEW 4 of 21 being filtered, washed, and dried. Finally, following calcination at 550 ◦ C for 6 h in air, ZnO-MCM was obtained. samples were that noted assamples ZnO-MCM-3, ZnO-MCM-9, and ZnO-MCM-0, whereand 3, 9,MCMand 0 where 3, 9,The and 0 represent the were prepared by Sal-MCM-3, Sal-MCM-9, represent that the samples were prepared by Sal-MCM-3, Sal-MCM-9, and MCM-41, respectively. 41, respectively.
Scheme 1. Schematic illustration of the synthetic process of ZnO-MCM. Scheme 1. Schematic illustration of the synthetic process of ZnO-MCM.
2.3. Photocatalytic Degradation of MO and Reusablity Study 2.3. Photocatalytic Degradation of MO and Reusablity Study The photoefficiency of ZnO-MCM was evaluated through the degradation of an MO aqueous The photoefficiency of ZnO-MCM was evaluated through the degradation of an MO aqueous solution under UV-light irradiation. In a typical experiment, 50 mg of ZnO-MCM was dispersed in solution under UV-light irradiation. In a typical experiment, 50 mg of ZnO-MCM was dispersed in 100 mL of a 10 mg/L (0.003 mmoL) MO solution. Then, the suspension was exposed to a UV lamp (300 W, 365 nm wavelength) with stirring. To quantify the progress of the photodegradation reaction, 3 mL of sampling was taken out at 15 min intervals using a 1 mL pipette and centrifuged at 12,000 r/min; then, a 0.2 μm Millipore membrane filter was used to completely remove the residual ZnOMCM nanoparticles to avoid its interference on subsequent MO absorbance analysis. A UV Lambda
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100 mL of a 10 mg/L (0.003 mmoL) MO solution. Then, the suspension was exposed to a UV lamp (300 W, 365 nm wavelength) with stirring. To quantify the progress of the photodegradation reaction, 3 mL of sampling was taken out at 15 min intervals using a 1 mL pipette and centrifuged at 12,000 r/min; then, a 0.2 µm Millipore membrane filter was used to completely remove the residual ZnO-MCM nanoparticles to avoid its interference on subsequent MO absorbance analysis. A UV Lambda 365 ultraviolet-visible spectrometer from Perkin Elmer (Waltham, MA, USA) was applied to measure the amount of MO degraded by ZnO-MCM. A linear regression for the relationship between the solution concentration (C) and absorbance (A) of the MO standard solutions at different concentrations was performed at λ = 463 nm to obtain a standard curvilinear equation: A = 0.06746C + 0.04755; R2 = 0.99906. The efficiency of the photocatalytic activity was calculated using the following equation: DA =
(C0 − Ct ) × 100% C0
(1)
where C0 is the original mass concentration (mg/L) of the MO aqueous solution, and Ct is the mass concentration (mg/L) of the MO aqueous solution at the relevant time interval. For contrast, the photocatalytic property of ZnO-MCM-0 and the self-degradation of MO were also examined. The reusability of a selected ZnO-MCM sample was examined. For this purpose, the photocatalyst was filtered off after reaction, washed several times using distilled water, centrifuged (5000 r/min for 15 min), and subsequently dried at 80 ◦ C in an oven. Then, it was reused as such for subsequent experiments (up to six cycles) under similar reaction conditions. For each cycle, the photocatalyst was reused for the photodegradation of a fresh MO solution. The same initial concentration of MO was used for each cycle (100 mL, 10 mg/L) and an irradiation time of 120 min was used. 2.4. Characterization The samples were analyzed using a Bruker AXS D8 X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5418 Å) and a graphite monochromator at 25 ◦ C, 40 kV, and 30 mA. The measurements were scanned at 2◦ /min (angular range 2θ = 0.5–5◦ and 5–90◦ ) in a 0.02◦ step size. The morphology of the particles was analyzed by a Spectrum 100 Fourier infrared spectrometer (Perkin-Elmer, Waltham, MA, USA) using the KBr squash technique. Gold particles were sprayed onto the surface of the samples under the protection of N2 , and the samples were characterized by a S4800 scanning electron microscope (Hitachi, Tokyo, Japan) to observe the surface topography. Transmission electron microscopy (TEM) observation was conducted on an FEI Tecnai G2 F20 transmission (Thermo Fisher Scientific, Waltham, MA, USA) election microscope. The Brunauer-Emmett-Teller (BET) surface area of the samples was determined by N2 adsorption isotherms at 77 K, operated on Quadrasorb SI adsorption equipment. The samples were degassed at 200 ◦ C for 12 h in vacuum before the N2 adsorption experiment. The zeta potential and particle size of the samples were investigated with a Zetasizer Nano ZS (Bruker Corporation, Karlsruhe, Germany) in water at pH = 7 through ultrasonic dispersion. X-ray photo electron spectroscopy (XPS) were recorded on an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA, Al Kα, hν = 1486.6 eV) under a vacuum of 2 × 10− 7 Pa. Charging effects were corrected by adjusting the main C 1s peak to a position of 284.8 eV. A TGA 2 thermogravimetric analyzer (Mettler-Toledo, Columbus, OH, USA) was used to analyze the heat stability of the particles over the heating range of 40~800 ◦ C and heating rate of 10 ◦ C/min. The diffuse reflectance UV absorbance spectra of samples were recorded at 25 ◦ C using a UV-3600 spectrophotometer (Shimadzu, Kyoto, Japan), all samples were measured with BaSO4 as the reference.
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1640
3. Results and Discussion
968
810
3413
3.1. Characterization
Sal-MCM 724
In Figure 1, Fourier-transform infrared spectroscopy (FTIR) was adopted to compare the different compositions of MCM-41, Sal-MCM, and ZnO-MCM. For MCM-41, the bands located at 3413 cm−1 1485 1237 2850 1630 − 1 and 968 cm were the stretching and bending vibrations of Si-OH, respectively [38]. The two bands 2929 ZnO-MCM that appeared at 1080 cm−1 and 810 cm−1 belonged to the characteristic peaks of Si-O-Si on the SiO2 1080 −1 and 2850 cm−1 framework. In comparison with MCM-41, two new bands appearing at 480 2929 cm of Sal-MCM were attributed to the symmetric and non-symmetric C-H stretching vibration bands from the aminopropyl group. The band 2000 of the 1500 benzene and the stretching band of 4000 3500 vibration 3000 2500 1000ring500 Wavenumber (cm-1) C=N in salicylaldimine were located at 780 cm−1 and 1630 cm−1 , respectively. It proved that the Figure 1.silica Fourier-transform infrared by spectroscopy (FTIR) spectra of MCM-41, Sal, Sal-MCM, mesoporous was well modified salicylaldimine. Compared with MCM-41, a redand shift at − 1 ZnO-MCM. 480 cm of ZnO-MCM was observed, which indicated that ZnO was well incorporated in the channels of MCM-41 [39]. As shown 2 adsorption/desorption isotherms of MCM-41, Sal-MCM-9, and As shown in in Figure Figure2a, 2a,the theNN 2 adsorption/desorption isotherms of MCM-41, Sal-MCM-9, Sal-MCM-3 belonged to the Langmuir IV type adsorption. Hysteresis loopsloops in these isotherms were and Sal-MCM-3 belonged to the Langmuir IV type adsorption. Hysteresis in these isotherms not obvious, which meant thatthat the the samples possessed a small pore size were not obvious, which meant samples possessed a small poresize size[40], [40],while whilethe the pore pore size distribution analyzed by the NLDFT method also confirmed the occurrence of micropores, as shown distribution analyzed by the NLDFT method also confirmed the occurrence of micropores, as shown in Figure Figure 2b. 2b. The adsorption and desorption branches branches of of Sal-MCM Sal-MCM could could not not be be easily easily duplicated duplicated in The adsorption and desorption within p/p p/poo ==0.0 o = 0.2 due to a certain extent of chemical adsorption by salicylaldimine during within 0.0to top/p p/p o = 0.2 due to a certain extent of chemical adsorption by salicylaldimine during the testing process [41]. addition, as shown in Table 1, 1, the the modification modification of of salicylaldimine salicylaldimine would would the testing process [41]. In In addition, as shown in Table significantly decrease decrease the the BET BET surface, surface, pore diameter, and and pore pore volume volume due due to to the the blocking blocking effect effect of of significantly pore diameter, 2/g and 2 salicylaldimine. Moreover, the BET surface of Sal-MCM-3 and Sal-MCM-9 were 453.385 m salicylaldimine. Moreover, the BET surface of Sal-MCM-3 and Sal-MCM-9 were 453.385 m /g and 634.437 m m22/g, salicylaldimine modification Sal-MCM-3 was was 634.437 /g,respectively, respectively, which which meant meant that that the the salicylaldimine modification on on Sal-MCM-3 more than than that that of ofSal-MCM-9. Sal-MCM-9.The Themodification modificationofofsalicylaldimine salicylaldiminenot not only decreased pore size more only decreased thethe pore size of of the mesoporous silica, but also broadened the distribution. the mesoporous silica, but also broadened the distribution.
a
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Figure 2. NN2 2adsorption/ adsorption/ desorption isotherms (a,b) pore distribution of MCM-41, Figure 2. desorption isotherms (a,b) andand pore sizesize distribution (c,d)(c,d) of MCM-41, SalSal-MCM, and ZnO-MCM. MCM, and ZnO-MCM.
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Table 1. The pore structural parameters of MCM-41, Sal-MCM, and ZnO-MCM. Materials
Surface Area (m2 /g)
Pore Diameter Dv (nm)
Pore Volume (m3 /g)
MCM ZnO-MCM-0 Sal-MCM-3 ZnO-MCM-3 Sal-MCM-9 ZnO-MCM-9
1187.377 625.657 453.385 739.543 634.437 848.967
2.444 0.887 1.934 2.180 1.932 2.176
0.993 0.429 0.388 0.547 0.478 0.682
As shown in Figure 2c, after the load of ZnO, the N2 adsorption/desorption isotherms of ZnO-MCM-0, ZnO-MCM-3 and ZnO-MCM-9 were still maintained as Langmuir IV types. Type H4 hysteresis loops were observed in the isotherms of ZnO-MCM-3 and ZnO-MCM-9 at p/po = 0.4, while the isotherm of ZnO-MCM-0 did not show any type of hysteresis loops. The appearance of the H4 hysteresis loop implied the instability of the adsorbed N2 due to the presence of ZnO in the pores of the mesoporous silica [40]. All the samples in Figure 2d showed three kinds of microporous distributions. Particularly, two mesopore systems, one with a relatively narrower and smaller pore size and another with a larger and broader pore size, were observed for ZnO-MCM-3 and ZnO-MCM-9. In contrast, ZnO-MCM-0 only exhibited a wider and smaller mesoporous distribution. It was supported that the introduction of ZnO would make the mesoporous silica form a hierarchical characteristic structure. As shown in Table 1, due to the blocking effect of ZnO, the BET surface, pore diameter, and pore volume decreased. Furthermore, when compared to MCM-41, the BET surface of ZnO-MCM-0, ZnO-MCM-3, and ZnO-MCM-9 decreased by about 561.72 m2 /g, 447.834 m2 /g, and 338.410 m2 /g, respectively. It can be seen that the more salicylaldimine modifications the MCM-41 underwent, the more zinc ions would be adsorbed so that more nano ZnO would be supported on MCM-41. Additionally, it was obvious that both the changes of the BET surface of ZnO-MCM-3 and ZnO-MCM-9 were less than that of ZnO-MCM-0. This phenomenon proved that using the chemical groups modified on mesoporous silica to adsorb zinc ions would cause less damage to the surface areas of the vehicle than that of the simple physical adsorption of pure mesoporous silica before oxidizing zinc ions to ZnO. Figure 3 shows the low angle X-ray diffraction (LXRD) patterns of MCM-41, Sal-MCM-3, Sal-MCM-9, ZnO-MCM-3, and ZnO-MCM-9. Three characteristic peaks are shown in MCM-41, which could be ascribed to the (100), (110), and (200) crystal faces, indicating that the particles had a regular hexagonal pore structure [42]. The modification of salicylaldimine made the intensity of the low-angle XRD peaks decrease, especially in the (110) and (200) crystal face. Furthermore, when the amount of salicylaldimine increased, all of the crystal faces disappeared. All these phenomena identified that salicylaldimine was introduced to the system and decreased its degree of orderliness [43]. After the load of ZnO, peaks of the (100) crystal face of both ZnO-MCM-3 and ZnO-MCM-9 shifted to a higher 2θ, which meant the occurrence of ZnO in the channels. In comparison to Sal-MCM-9, the intensity of the (100) crystal face peak of ZnO-MCM-9 decreased, and the peaks of the (110) and (200) crystal faces faded away. However, the pattern of ZnO-MCM-3 still maintained the same shape as that of Sal-MCM-3. This result proved that the degree of orderliness of salicylaldimine-modified MCM-41 is crucial for ZnO formation inside the channels, and it further affects the orderliness of ZnO-MCM. Figure 3 also shows the wide-angle X-ray diffraction patterns (WXRD) of MCM-41, ZnO-MCM-0, ZnO-MCM-3, and ZnO-MCM-9. Compared with MCM-41, no characteristic peaks of ZnO were observed in these wide-angle patterns of ZnO-MCM-0 and ZnO-MCM-3. Only diffuse peaks of the non-crystalline silica appeared, meaning that ZnO was finely loaded on the mesoporous silica and the cluster size of ZnO was too small to be detected by X-ray [44]. However, it also confirmed that ZnO was not present in the crystalline form [45], and the growth of ZnO was limited by the mesostructure of the vehicle. For ZnO-MCM-9, although the LXRD showed that the material was not in a good degree of orderliness, a series of peaks were still observed at the (100), (002), (101), and (110) crystal
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faces, which could be indexed to the wurtzite structure of ZnO [46]. Combined with the blocking effect blocking effect confirmed by the N2 adsorption/desorption, meant thatof the bad degree orderliness confirmed by the N2 adsorption/desorption, it meant that theitbad degree orderliness ofof ZnO-MCM-9 of ZnO-MCM-9 wasFrom caused ZnO. From thefrom above from the supported XRD patterns, was was caused by ZnO. the by above information theinformation XRD patterns, it was that aitgood supported that a good degree of orderliness of the vehicle as well as an appropriate amount of ZnO degree of orderliness of the vehicle as well as an appropriate amount of ZnO could help ZnO form helpstructure ZnO form wurtzite structure acould wurtzite ona mesoporous silica. on mesoporous silica. 4000
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Figure 3. Low-X-ray diffraction patterns (XRD) (a) and Wide-XRD (b) patterns of MCM-41, Sal-MCM, ZnO-MCM. and ZnO-MCM.
As listed listed in in Table Table 2, 2, the the zeta zeta potential potential of ofSal-MCM-3 Sal-MCM-3 shifted shifted from from − −35.38 owing to to As 35.38 to to 40.08 40.08 mV, mV, owing the positive positive ions ions from fromthe thenitric nitricofofsalicylaldimine salicylaldimine[47]. [47].The Theresult resultproved provedthat that the surface MCMthe the surface of of MCM-41 41 was modified with salicylaldimine. Table 2 also shows that the zeta potential of ZnO-MCM-3 was modified with salicylaldimine. Table 2 also shows that the zeta potential of ZnO-MCM-3 shifted shifted fromto −35.38 to mV. −23.08 mV. It that proved that positively charged ZnO nanoparticles could partly from −35.38 −23.08 It proved positively charged ZnO nanoparticles could partly neutralize neutralize the negative electric charges on the MCM-41 surface. However, Hassan [48] found that the negative electric charges on the MCM-41 surface. However, Hassan [48] found that after the ZnO after the ZnO was supported on MCM-41, the material showed a positive zeta potential, which was was supported on MCM-41, the material showed a positive zeta potential, which was about 9.3 mV. about 9.3 mV. According the detection mechanism ofusually zeta potential, usually the electric potential According to the detectiontomechanism of zeta potential, the electric potential detected by the detected by the machine was from the electrostatic field of the surface of the nanoparticles [49]. machine was from the electrostatic field of the surface of the nanoparticles [49]. This confirmed This that confirmed of the ZnO nanoparticles were loadedofinMCM-41 the channels of MCM-41 andofonly most of the that ZnOmost nanoparticles were loaded in the channels and only a small part ZnOa smallloaded part ofon ZnO loaded on thewhich outside surface, couldfrom also be the change of was thewas outside surface, could alsowhich be proved theproved changefrom of pore diameter pore diameter shown in Table 1. shown in Table 1. Table Table 2. 2. Zeta potential potential of of MCM-41, MCM-41, Sal-MCM, Sal-MCM, and and ZnO-MCM. ZnO-MCM.
Materials Materials
Zeta Potential (mV)
Zeta Potential (mV)
MCM-41 MCM-41 −35.38
−35.38
Sal-MCM-3 Sal-MCM-3 40.08 40.08
ZnO-MCM-3 ZnO-MCM-3 −23.08 −23.08
Figures 4 and 5 depict the SEM and TEM images of MCM-41, Sal-MCM, ZnO-MCM-0, and ZnOFigures and 5 depict the SEM and TEMpore images of MCM-41, Sal-MCM, ZnO-MCM-0, MCM-9. As 4 shown, the regular hexagonal structure was well-maintained without and ZnO-MCM-9. As shown, the regular hexagonal pore structure was well-maintained without agglomeration after the modification of salicylaldimine or the load of ZnO. Both Sal-MCM and ZnOagglomeration after the modification of salicylaldimine or the load of ZnO. Both Sal-MCM MCM showed a rough shell structure on the external surface with many circular particles due toand the ZnO-MCM a rough shell structure on surface the external surface with many 4d) circular due introductionshowed of salicylaldimine and ZnO. The of ZnO-MCM-9 (Figure was particles rougher than to theofintroduction salicylaldimine and ZnO. surface of ZnO-MCM-9 (Figure 4d) was rougher that ZnO-MCM-0of(Figure 4c), indicating that The salicylaldimine could help the vehicles adsorb more than that of ZnO-MCM-0 (Figure 4c), indicating that salicylaldimine could help the vehicles adsorb zinc ions and, thus, more ZnO would form. Comparing the wurtzite form of nano ZnO showed by more zinc and, thus,form morenanoparticles ZnO would form. Comparing wurtzite form of nano ZnO showed WXRD to ions the spherical on the surface ofthe ZnO-MCM-9, it was obvious that the by WXRD to the spherical form nanoparticles on the surface of ZnO-MCM-9, it was obvious that wurtzite shaped ZnO were formed in the channels instead of the outside surface of MCM-41. This the wurtzite shaped ZnO were formed in the channels instead of the outside surface of MCM-41. proved that the shape of ZnO nanoparticles could be controlled by the changeable channels of This proved silica, that the shape of ZnO ZnO nanoparticles nanoparticlesgrown could on be the controlled by the changeable channels mesoporous while those outside surface could not form in the of mesoporous silica, while those ZnO nanoparticles grown on the outside surface could not form same shape due to the lack of restrictions. Materials in Figure 5 all presented in the parallel lattice in the same shape due to the lack of restrictions. Materials in Figure 5 all presented in the parallel fringes structure [50] and corresponded to (100) crystal face, which showed the highest peak in Lowlattice XRD. fringes structure [50] and corresponded to (100) crystal face, which showed the highest peak in Low-XRD.
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Figure 4. Scanning Electron Microscope (SEM) images of MCM-41 (a); Sal-MCM (b); ZnO-MCM-0 (c); Figure 4. Scanning Electron Microscope (SEM) images of MCM-41 (a); Sal-MCM (b); ZnO-MCM-0 (c); Figure 4. Scanning(d). Electron Microscope (SEM) images of MCM-41 (a); Sal-MCM (b); ZnO-MCM-0 (c); and ZnO-MCM-9 and ZnO-MCM-9 (d). and ZnO-MCM-9 (d).
Figure 5. Cont.
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Figure 5. electron microscopy (TEM) images of MCM-41 (a,b), (a,b), Sal-MCM (c,d), ZnO5. Transmission Transmission electron microscopy (TEM) images of MCM-41 Sal-MCM (c,d), MCM-0 (e,f), and (g,h).(g,h). ZnO-MCM-0 (e,f),ZnO-MCM-9 and ZnO-MCM-9
As As seen seen in in Figure Figure 6, 6, the the XPS XPS analysis analysis was wascarried carriedout outto toinvestigate investigatethe thenanoparticle nanoparticlecomposition. composition. In Figure 6a, it was observed that the binding energy regions positioned at 1022 In Figure 6a, it was observed that the binding energy regions positioned at 1022 eV eV and and 1045 1045 eV eV were were assigned to Zn 2p 3/2 and Zn 2p1/2, respectively [51], from the spectrum lines of both of the ZnO-MCMassigned to Zn 2p3/2 and Zn 2p1/2 , respectively [51], from the spectrum lines of both of the ZnO-MCM-3 3and andZnO-MCM-9. ZnO-MCM-9.Zn/Sal-MCM-3 Zn/Sal-MCM-3and andZn/Sal-MCM-9 Zn/Sal-MCM-9 showed and showed two two peaks peaks of of Zn Zn 2p 2p at at 1021 1021 eV eV and 1044 eV, respectively, which means that the zinc ions were coordinated with salicylaldimine. After 1044 eV, respectively, which means that the zinc ions were coordinated with salicylaldimine. After the the formation of ZnO, the peak at 1044 eVZn of2p Zn 2pwas 1/2 was more obvious. Meanwhile, the peak at 1021 formation of ZnO, the peak at 1044 eV of more obvious. Meanwhile, the peak at 1021 eV 1/2 eV shifted positively to 1022 eV, due to the coordination bonds between zincand ionssalicylaldimine and salicylaldimine shifted positively to 1022 eV, due to the coordination bonds between zinc ions being being replaced by the ionic bonds between the zinc atoms and oxygen atoms. On the one hand, the replaced by the ionic bonds between the zinc atoms and oxygen atoms. On the one hand, the spectrum spectrum of ZnO-MCM-0 showed no special difference from that of MCM-41, while the spectrum of ZnO-MCM-0 showed no special difference from that of MCM-41, while the spectrum lines of lines of ZnO-MCM-3 and ZnO-MCM-9 were distinctly from of Sal-MCM-3 and SalZnO-MCM-3 and ZnO-MCM-9 were distinctly differentdifferent from those ofthose Sal-MCM-3 and Sal-MCM-9. MCM-9. On the otherfor hand, for ZnO-MCM-0, its atomic concentration was while 0.39, while ZnO-MCM-3 On the other hand, ZnO-MCM-0, its atomic concentration was 0.39, ZnO-MCM-3 and and ZnO-MCM-9 showed the atomic concentrations of 2.03 and 1.27, respectively (Table 3).results These ZnO-MCM-9 showed the atomic concentrations of 2.03 and 1.27, respectively (Table 3). These results confirmed the more ZnO was on loaded on MCM-41, more obvious the peaksbe. would be. confirmed that thethat more ZnO was loaded MCM-41, the morethe obvious the peaks would It could It be concluded the modification of salicylaldimine doestohelp to more catch zinc moreions zincdue ionstodue becould concluded that thethat modification of salicylaldimine does help catch its to its strong coordination function. From Figure 7, we also found that the binding energy of the Zn strong coordination function. From Figure 7, we also found that the binding energy of the Zn 3d 3d electron in ZnO-MCM is 11.58 eV, 1.93 eV higher than that of pure ZnO, which indicates the electron in ZnO-MCM is 11.58 eV, 1.93 eV higher than that of pure ZnO, which indicates the formation formation of Si-O-Zn bond. This proved that, after the calcination, the interaction between ZnO and of Si-O-Zn bond. This proved that, after the calcination, the interaction between ZnO and the silica the silica matrix is the Si-O-Zn bond [39]. matrix is the Si-O-Zn bond [39]. Figure 6b exhibits the binding energy spectrum line of N 1s. A positive shift of Zn/Sal-MCM-3 Table 3. Binding energy and atomic of MCM-41, Sal-MCM, Zn/Sal-MCM, and ZnO-MCM. was observed, which showed that the peak value of 399.44 eV moved to 400.14 eV. It was proposed BE/eV that the electron transfer from nitrogen to zinc ions should be responsibleAtomic/% for this result [52]. It proved Material that the coordination between theZn salicylaldimine was successfully formed. 2p N 1s and Si zinc 2p ions O 1s Zn 2p Zn 2p3s MCM 103.83 533.19 Sal-MCM-3 399.44 102.63 532.20 Sal-MCM-9 401.87 102.47 532.09 Zn/Sal-MCM-3 1021.26 400.14 102.40 532.17 0.334
ZnO-MCM-9
1022.90
-
103.29
532.67
1.27
-
Figure 6b exhibits the binding energy spectrum line of N 1s. A positive shift of Zn/Sal-MCM-3 was observed, which showed that the peak value of 399.44 eV moved to 400.14 eV. It was proposed that the electron transfer from nitrogen to zinc ions should be responsible for this result [52]. It proved Nanomaterials 2018, 8, 317 11 of 21 that the coordination between the salicylaldimine and zinc ions was successfully formed. a
30,000
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Figure 6. X-ray photo electron spectroscopy (XPS) of Zn 2p (a); N 1s (b); O 1s (c); Si 2p (d); survey (e,f) Figure 6. X-ray photo electron spectroscopy (XPS) of Zn 2p (a); N 1s (b); O 1s (c); Si 2p (d); survey (e,f) ofMCM-41, ofMCM-41, Sal-MCM, Sal-MCM, and and ZnO-MCM. ZnO-MCM. Table 3. Binding energy and atomic of MCM-41, Sal-MCM, Zn/Sal-MCM, and ZnO-MCM. BE/eV
Material MCM Sal-MCM-3 Sal-MCM-9 Zn/Sal-MCM-3 Zn/Sal-MCM-9 ZnO-MCM-0 ZnO-MCM-3 ZnO-MCM-9
Atomic/%
Zn 2p
N 1s
Si 2p
O 1s
Zn 2p
Zn 2p3s
1021.26 1021.75 1017.90 1022.27 1022.90
399.44 401.87 400.14 401.14 -
103.83 102.63 102.47 102.40 102.54 103.81 102.84 103.29
533.19 532.20 532.09 532.17 532.24 533.18 532.19 532.67
0.39 2.03 1.27
0.334 0.415 -
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Zn3d ZnO-MCM 11.58 eV
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Figure 7. 7. XPS XPS of of Zn Zn 3d 3d of of ZnO-MCM. ZnO-MCM. Figure
In Figure 6c, all the binding energy peaks of O 1s were symmetrical, which was different from In Figure 6c, all the binding energy peaks of O 1s were symmetrical, which was different from previous reports [50] that indicated that only one chemical state for the oxygen species existed. For previous reports [50] that indicated that only one chemical state for the oxygen species existed. MCM-41, a peak centered at 533.19 eV was definitely from the result of the oxygen atoms of MCMFor MCM-41, a peak centered at 533.19 eV was definitely from the result of the oxygen atoms of 41. Those energy peaks of O 1s, excluding that of ZnO-MCM-0, took a negative shift in varying MCM-41. Those energy peaks of O 1s, excluding that of ZnO-MCM-0, took a negative shift in varying degrees after the modification of salicylaldimine or ZnO. This means that the modification of degrees after the modification of salicylaldimine or ZnO. This means that the modification of chemical chemical groups or metal oxides would disturb the electron distribution of the surface of the vehicles. groups or metal oxides would disturb the electron distribution of the surface of the vehicles. After the After the ZnO formed on the MCM-41, the electron intensity of O 1s also increased, when compared ZnO formed on the MCM-41, the electron intensity of O 1s also increased, when compared to those to those of Sal-MCM-3 and Sal-MCM-9. It could be suggested that the zinc ions were transformed of Sal-MCM-3 and Sal-MCM-9. It could be suggested that the zinc ions were transformed into ZnO into ZnO because during this process, oxygen atoms would combine with zinc atoms, resulting in because during this process, oxygen atoms would combine with zinc atoms, resulting in the increase the increase of intensity of O 1s. In Figure 6d, the same change trend was also observed from the of intensity of O 1s. In Figure 6d, the same change trend was also observed from the binding energy binding energy spectrum of Si 2p, which could also be attributed to the similar reason. The overall spectrum of Si 2p, which could also be attributed to the similar reason. The overall XPS spectrum XPS spectrum of MCM-41, ZnO-MCM, Sal-MCM, and Zn/Sal-MCM are shown in Figure 6e,f. All of of MCM-41, ZnO-MCM, Sal-MCM, and Zn/Sal-MCM are shown in Figure 6e,f. All of the spectrum the spectrum results confirmed that ZnO was successfully loaded onto MCM-41. results confirmed that ZnO was successfully loaded onto MCM-41. The UV-vis DRS of the nano-zinc oxide, mesoporous silica, and ZnO-MCM samples are shown The UV-vis DRS of the nano-zinc oxide, mesoporous silica, and ZnO-MCM samples are shown in in Figure 8. The adsorption band edge at 372 nm for ZnO-MCM suggests the presence of ZnO Figure 8. The adsorption band edge at 372 nm for ZnO-MCM suggests the presence of ZnO particles [6]. particles [6]. Compared with ZnO, a red shift of the adsorption band edge was observed, which can Compared with ZnO, a red shift of the adsorption band edge was observed, which can be ascribed to be ascribed to the well-known quantum size effect [53]. The wide absorption band from 250 to 372 the well-known quantum size effect [53]. The wide absorption band from 250 to 372 nm observed in nm observed in the ZnO-MCM sample might be due to the formation of one-dimensional array inside the ZnO-MCM sample might be due to the formation of one-dimensional array inside the mesoporous the mesoporous silica, which is expected for these types of crystal growth [39]. Based on the silica, which is expected for these types of crystal growth [39]. Based on the maximum absorption maximum absorption wavelength, the band gap of ZnO nanoparticles supported on mesoporous wavelength, the band gap of ZnO nanoparticles supported on mesoporous silica was calculated to silica was calculated to be 3.33 eV according to the relation Ebg = 1240/λmax, while the band gap of pure be 3.33 eV according to the relation Ebg = 1240/λmax , while the band gap of pure nano-zinc oxide nano-zinc oxide was 3.50 eV [54]. This means that the photocatalytic performance of ZnO was was 3.50 eV [54]. This means that the photocatalytic performance of ZnO was improved by being improved by being supported on mesoporous silica. supported on mesoporous silica.
Absorbance Absorbance
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MCM-41 ZnO-MCM MCM-41 ZnO ZnO-MCM ZnO
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400 500 600 400Wavenumber/nm 500 600
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Wavenumber/nm Figure 8. Diffuse reflectance UV-vis spectra of MCM-41, ZnO-MCM, and ZnO. Figure spectra of of MCM-41, MCM-41, ZnO-MCM, ZnO-MCM, and and ZnO. ZnO. Figure 8. 8. Diffuse Diffuse reflectance reflectance UV-vis UV-vis spectra
As shown in Figure 9, ZnO-MCM-3 and ZnO-MCM-9 were subjected to TGA to obtain the As shown in Figure 9, ZnO-MCM-3 and ZnO-MCM-9 were subjected to in TGA to from obtain thermal stability For both and ZnO-MCM-3 andwere ZnO-MCM-9, theTGA loss mass 40the °C As shown in information. Figure 9, ZnO-MCM-3 ZnO-MCM-9 subjected to to obtain the thermal thermal stability information. For both ZnO-MCM-3 and ZnO-MCM-9, the loss in mass from 40 °C ◦ ◦ to 100 °Cinformation. was due to For the both elimination of the and crystallization water physically stability ZnO-MCM-3 ZnO-MCM-9, theand loss residual in mass from 40 C adsorbed to 100 C to 100 on °C the wassurface due toofthe elimination of the crystallization water and residual physically adsorbed water the channels [55]. The final residues of ZnO-MCM-3 and ZnO-MCM-9 were was due to the elimination of the crystallization water and residual physically adsorbed water on the water on 91%, the surface of the which channels [55]. The final residues of ZnO-MCM-3 and supported ZnO-MCM-9 were 86% and respectively, showed that there were more nano zinc oxides on ZnOsurface of the channels [55]. The final residues of ZnO-MCM-3 and ZnO-MCM-9 were 86% and 91%, 86% andthan 91%,that respectively, which showed that there moresalicylaldimine nano zinc oxides supported onMCMZnOMCM-3 ZnO-MCM-9. It also thatwere the was grafted on respectively, whichofshowed that there wereproved more nano zincmore oxides supported on ZnO-MCM-3 than that MCM-3 than that of ZnO-MCM-9. It also proved that the more salicylaldimine was grafted on MCM41, the more zinc ions would be coordinated and thus, more nano zinc oxides would form. According of ZnO-MCM-9. It also proved that the more salicylaldimine was grafted on MCM-41, the more zinc 41, theprevious more zinc ions would be coordinated and thus, more zinc oxides would form. According to the literature [56], ZnOmore transformed Zn2nano SiO 4 began from 650 to 800 to the ions would be coordinated andthe thus, nano zincinto oxides would form. According to °C thedue previous to the previous literature [56], the ZnO transformed into Zn 2 SiO 4 began from 650 to 800 °C due to the ◦ C due reaction between ZnOtransformed and SiO2. However, in 4this work, curves didtonot an between obvious literature [56], the ZnO into Zn2 SiO began fromthe 650TG to 800 theshow reaction reaction between ZnO and SiO 2 . However, in this work, the TG curves did not show an obvious sudden step at this temperature range and only a slow loss of weight during 100 to 800 °C ZnO andheat SiOloss 2 . However, in this work, the TG curves did not show an obvious sudden heat loss step sudden heat loss step at this temperature range and only slow loss of began weightatduring 100 to 800 °C was observed. It is range supposed transformation of athis material 100 observed. °C and only at this temperature and that onlythe a slow loss of weight during 100 to 800 ◦ C was It isa was observed. It is supposed that the transformation of this material began at 100 °C and only a fraction ofthat ZnO Further research on began this phenomenon in our future supposed thetransform. transformation of this material at 100 ◦ C andwill onlybea undertaken fraction of ZnO transform. fraction of ZnO transform. Further research on this phenomenon will be undertaken in our future work. research Further on this phenomenon will be undertaken in our future work. work. 100
ZnO-MCM-3 ZnO-MCM-3 ZnO-MCM-9 ZnO-MCM-9
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Figure 9. Thermogravimetry of ZnO-MCM-3 ZnO-MCM-3 and and ZnO-MCM-9. ZnO-MCM-9. Figure 9. Thermogravimetry (TGA) (TGA) curves curves of Figure 9. Thermogravimetry (TGA) curves of ZnO-MCM-3 and ZnO-MCM-9.
3.2. Degradation 3.2. Degradation of of MO MO 3.2. Degradation of MO Figure 10 activity of of thethe as-prepared ZnO-MCM-0, ZnO-MCM-3, and Figure 10 shows showsthe thephotocatalytic photocatalytic activity as-prepared ZnO-MCM-0, ZnO-MCM-3, Figure 10nanoparticles. shows the photocatalytic activity of the as-prepared ZnO-MCM-0, ZnO-MCM-3, and ZnO-MCM-9 The photodegradation of the MO was investigated in deionized water and ZnO-MCM-9 nanoparticles. The photodegradation of the MO was investigated in deionized ZnO-MCM-9 nanoparticles. The of theadding MO was innanoparticles deionized water under under ultraviolet irradiation. Thephotodegradation MO without theinvestigated as-prepared was water ultraviolet irradiation. Thesolution MO solution without adding the as-prepared nanoparticles under ultraviolet irradiation. The MO solution without adding the as-prepared nanoparticles chosen for comparison. The degradation process can be divided into two parts. During 45 min,was the chosen for comparison. The degradation process can be divided into two parts. During 45 min, the
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was chosen for comparison. The degradation process can be divided into two parts. During 45 min, the double effect of adsorption and degradation of ZnO-MCM-9 and ZnO-MCM-3 double effect of adsorption and degradation of ZnO-MCM-9 and ZnO-MCM-3 mademade the the concentration of MO rapidly decline by 54% respectively. From themin 45 min to end, the end, concentration of MO rapidly decline by 54% andand 45%,45%, respectively. From the 45 to the the the degradation rate caused by ZnO-MCM-3 was than fasterthat than that of ZnO-MCM-9. This could be degradation rate caused by ZnO-MCM-3 was faster of ZnO-MCM-9. This could be indicated indicated that in the first process of photocatalytic degradation, the surface area played a dominant that in the first process of photocatalytic degradation, the surface area played a dominant role, while role, in the second of ZnO loaded ondominated MCM-41 dominated the degradation in thewhile second process, theprocess, amountthe of amount ZnO loaded on MCM-41 the degradation according according to the zinc atomic weight of ZnO-MCM-3, which was more than that of ZnO-MCM-9 to the zinc atomic weight of ZnO-MCM-3, which was more than that of ZnO-MCM-9 while the BET while surface of ZnO-MCM-3 smaller. Furthermore, of orderliness surfacetheofBET ZnO-MCM-3 was smaller.was Furthermore, the degreethe of degree orderliness as well as aswell the as the crystallinity of the zincdid oxide did not show any significant on the photocatalytic crystallinity of the zinc oxide not show any significant influenceinfluence on the photocatalytic efficiency efficiency of ZnO-MCM. a result, it took approximately 120 minthe to MO reduce the MO concentration of ZnO-MCM. As a result,As it took approximately 120 min to reduce concentration using ZnOusing ZnO-MCM-3, while an additional 30 min was required with of ZnO-MCM-9. The pure MCM-3, while an additional 30 min was required with the usethe of use ZnO-MCM-9. The pure MO MO degraded approximately after min, whichimplied impliedthat thatwithout without ZnO-MCM, ZnO-MCM, ultraviolet degraded approximately 35%35% after 200200 min, which radiation only caused negligible negligible degradation degradation to to MO. MO.
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Figure 10. 10.Photodegradation Photodegradationofof MO under ZnO-MCM-0, ZnO-MCM-3 and ZnOFigure MO under UV UV lightlight usingusing ZnO-MCM-0, ZnO-MCM-3 and ZnO-MCM-9 MCM-9 (a); andabsorbance UV-Vis absorbance of MO (b). (a); and UV-Vis spectra ofspectra MO (b).
The two photos inset in Figure 10a reflect the obvious change in the color of the MO solution The two photos inset in Figure 10a reflect the obvious change in the color of the MO solution with the addition of ZnO-MCM and the ultraviolet adsorption spectrum was used to confirm the with the addition of ZnO-MCM and the ultraviolet adsorption spectrum was used to confirm the degradation of MO. The color changed from orange to colorless in 150 min with the use of ZnO-MCM degradation of MO. The color changed from orange to colorless in 150 min with the use of ZnO-MCM under UV light, and the characteristic peak at 463 nm for the absorption became weaker and under UV light, and the characteristic peak at 463 nm for the absorption became weaker and disappeared in the final. This observation indicated that ZnO-MCM would accelerate the degradation disappeared in the final. This observation indicated that ZnO-MCM would accelerate the degradation of MO. of MO. 3.3. Degradation Kinetics Study 3.3. Degradation Kinetics Study According to previous reports [39,50,57,58], a pseudo-second-order model (Equation (4)) [59] According to previous reports [39,50,57,58], a pseudo-second-order model (Equation (4)) [59] and and pseudo-first-order model (Equation (3)) [60–62] evolved from the Langmuir−Hinshelwood pseudo-first-order model (Equation (3)) [60–62] evolved from the Langmuir−Hinshelwood mechanism mechanism (L-H model, Equation (2)) [63–65] can be applied to describe the photocatalytic (L-H model, Equation (2)) [63–65] can be applied to describe the photocatalytic degradation process of degradation process of the organic dye reaction at the liquid-solid interface [66]. All the equations the organic dye reaction at the liquid-solid interface [66]. All the equations were displayed as follows: were displayed as follows: dC Kr K s C r = dC= K r K s C (2) r dt 1 + Ks Co (2)
dt
1 K s Co
At a low initial concentration of MO, Equation (2) can be transformed into Equation (3). At a low initial concentration of MO, Equation (2) can be transformed into Equation (3). Co ln C = Kapp t Cto
ln K app t Ct
(3) (3)
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1
1 Kt 1 = Kt + C C Ctt Coo
(4) (4)
where represent the where C Coo and and C Ctt represent the concentration concentration at at the the initial initial and and treatment treatment process process time. time. K Kss and and K Krr are are the adsorption rate and reaction rate, respectively. K and K are the apparent pseudo-first-order and app the adsorption rate and reaction rate, respectively. Kapp and K are the apparent pseudo-first-order and second-order second-order rate rate constants, constants, respectively. respectively. The dataof the of MO the degradation MO degradation were fitted to a pseudo-first-order model and The data were fitted to a pseudo-first-order model and a pseudo-seconda pseudo-second-order model (Figure 11). The R values and constants of the two order model (Figure 11). The R values and constants of the two models are listed in Table 4.models It was are listed in the Table 4. It was obvious the pseudo-first-order model was more suitable obvious that pseudo-first-order model that was more suitable than the pseudo-second-order model than theMO pseudo-second-order model for the MO degradation. For ZnO-MCM-0, for the degradation. For ZnO-MCM-0, ZnO-MCM-9, and ZnO-MCM-3, the KappZnO-MCM-9, values were and ZnO-MCM-3, the K values were 0.00565, 0.01816 and 0.02491, respectively, which that app 0.00565, 0.01816 and 0.02491, respectively, which indicated that the degradation rate wasindicated increasingly the degradation rate was increasingly faster as the amount of ZnO increased.faster as the amount of ZnO increased.
a 3.2
b2.0 2.2
ZnO-MCM-0 ZnO-MCM-3 ZnO-MCM-9
2.8 2.4
1.8 1.6 1.4
2.0
1.2
1/Ct
ln(Co/Ct)
ZnO-MCM-0 ZnO-MCM-3 ZnO-MCM-9
1.6
1.0
1.2
0.8
0.8
0.6 0.4
0.4 0.0 0
0.2 10
20
30
40
50
60
Time (min)
70
80
90
100
110
0.0 0
10
20
30
40
50
60
Time (min)
70
80
90
100
110
Figure 11. Photodegradation kinetics of MO using the pseudo-first-order model (a); and the pseudoFigure 11. Photodegradation kinetics of MO using the pseudo-first-order model (a); and the second-order model (b). pseudo-second-order model (b). Table 4. Fitting results of the photodegradation of MO. Table 4. Fitting results of the photodegradation of MO.
Kinetic Models Kinetic Models
pseudo first-order
pseudo first-order
pseudo second-order pseudo second-orderC1t
Fitting Equation Fitting Equation t ; ZnO-MCM-0 0.00565 Co 0.00565t; ln 0.02491ZnO-MCM-0 t ; ZnO-MCM-3 Ct 0.02491t; ZnO-MCM-3 ln CCot = 0.01816t ; ZnO-MCM-9 0.01816t; ZnO-MCM-9 0.0007t 0.09882; ZnO-MCM-0 1 0.0007t + 0.09882; ZnO-MCM-0 ZnO-MCM-3 = 0.01094t 0.01094+t 0.10335; 0.10355; ZnO-MCM-3 Ct 0.00472t + 0.10335; ZnO-MCM-9 0.00472t 0.10355; ZnO-MCM-9
R2 R2 0.98225 0.98225 0.97787 0.97787 0.99306
0.99306
0.99913 0.99913 0.74946 0.74946 0.93623 0.93623
3.4. Degradation Mechanism Mechanism 3.4. Degradation A A possible possible mechanism mechanism of of the the degradation degradation behavior behavior would would help help to to better better understand understand the the photocatalysis result (Scheme 2). As previous reports have depicted [67], irradiation photocatalysis result (Scheme 2). As previous reports have depicted [67], irradiation with with aa suitable suitable UV-light UV-light source source whose whose band band gap gap energy energy is is smaller smaller than than its its photon photon energy energy would would activate activate the the photocatalyst [68]. When a photon is caught by zinc oxide, it facilitates an electron transfer from photocatalyst [68]. When a photon is caught by zinc oxide, it facilitates an electron transfer from the the valence the conduction this moment, an electron-hole pair emerges. These valence bandband to thetoconduction band.band. At thisAt moment, an electron-hole pair emerges. These electronelectron-hole pairsto migrate to theofsurface of ZnO start toand oxidize and the MO molecules hole pairs migrate the surface ZnO and start and to oxidize reduce thereduce MO molecules absorbed absorbed on the outside surface or in the channels of the ZnO-MCM [69]. The surface hydroxyl on the outside surface or in the channels of the ZnO-MCM [69]. The surface hydroxyl groups acquire groups acquire the holes and strongly to radicals, form hydroxyl whichinare generated the holes and strongly oxidize to form oxidize hydroxyl which radicals, are generated this process. in Atthis the process. At the same time, the combined materials, which include the aqueous solution, the oxide same time, the combined materials, which include the aqueous solution, the oxide and the oxide and the oxide produce hydroxyl radicals. hydroxyl Ultimately, hydroxyl radicals by generated by radicals, also radicals, produce also hydroxyl radicals. Ultimately, radicals generated these two
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processes allow the oxidation of the MO dye absorbed on the surface of ZnO, leading to the formation 16 of 21 of ammonium, nitrate ion, sulfate, and CO2.
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these two processes allow the oxidation of the MO dye absorbed on the surface of ZnO, leading to the processes allow the oxidation of the MO dye absorbed on the surface of ZnO, leading to the formation formation of ofammonium, ammonium, nitrate ion, sulfate, nitrate ion, sulfate, and CO2.and CO2 .
Scheme 2. Schematic illustration of degradation mechanism.
Scheme 2. Schematic illustration of degradation mechanism. 3.5. Reusability Performance
3.5. Reusability Performance 3.5. ReusabilityAs Performance shown in Figure 12, Tables 5 and 6, after six cycles, ZnO-MCM-3 was still effective in the photodegradation MO, remaining above the photocatalyticwas activity be As shown shown in Figure Figure of12, 12, Tables and 6, 70%. afterThe sixdecrease cycles,inZnO-MCM-3 ZnO-MCM-3 stillmight effective in the the As in Tables 55 and 6, after six cycles, was still effective in due to the loss of zinc, the change of pore structure and the reduction of surface area. These change photodegradation of MO, remaining above 70%. The decrease in the photocatalytic activity might be photodegradation of MO, remaining 70%. of The the photocatalytic of parameters not only resulted above in the change thedecrease amount ofin active oxygen produced activity by ZnO- might be due to the loss of zinc, the change of pore structure and the reduction of surface area. These change but also a decrease the contact areathe between ZnO-MCM-3 and MO. of the change of due to the MCM-3, loss of zinc, thebrought change of poreofstructure and reduction of surface area.AllThese factors ultimately affected the photodegradation efficiency. of parameters not only resulted in the change of the amount of active oxygen produced by ZnOparameters not only resulted in the change of the amount of active oxygen produced by ZnO-MCM-3, MCM-3, but also brought a decrease of the contact area between ZnO-MCM-3 and MO. All of the but also brought a decrease of the contact area between ZnO-MCM-3 and MO. All of the factors a b factors ultimately the photodegradation efficiency. Zn 2p ZnO-MCM-3 reusability ultimately affectedaffected the photodegradation efficiency. 100 90
100 90
Photodegradation/%
80 70
Zn 2p
50
ZnO-MCM-3 reusability
40 30
10
50
0
40
15 14 13 12
2
3
4
5
d
13
1
1020
1025
1030
1035
1040
1045
1050
Binding Energy/eV
0.025
ZnO-MCM-3 reusability 0.020
12 11 10 9
0.015
2
3
4
5
6
Reaction cycle
d
8
ZnO-MCM-3 reusability 7
1015
0.005
5
0.020
4 0.0
0.2
0.4
0.6
0.8
1.0
P/Po
1020
1025
0.000
1030
1035
1040
1045
1050
Binding Energy/eV
0.010
0.025
6
ZnO-MCM-3 reusability 2
4
6
Pore width
8
10
12
dV(d)
0.015
10
8
1015
ZnO-MCM-3 reusability
14
11
9
6
15
Volume @ STP(cc/g)
0
1
Reaction cycle
c
30
10
Volume @ STP(cc/g)
b
60
20
60
20
c
70
dV(d)
a
Photodegradation/%
80
Figure 12. Reusability of ZnO-MCM-3 toward the photodegradation of MO (a), XPS of Zn 2p (b), N2 0.010 adsorption/desorption isotherms (c) and pore size distribution (d).
7 0.005
6 5 4 0.0
0.2
0.4
0.6
P/Po
0.8
1.0
0.000
2
4
6
Pore width
8
10
12
Figure Figure 12. 12. Reusability Reusability of of ZnO-MCM-3 ZnO-MCM-3 toward toward the the photodegradation photodegradation of of MO MO (a), (a), XPS XPS of of Zn Zn 2p 2p (b), (b), N N22 adsorption/desorption isotherms (c) and pore size distribution (d). adsorption/desorption isotherms (c) and pore size distribution (d).
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Table 5. The pore structural parameters of ZnO-MCM-3 after six photodegradation cycles of MO. Materials
Surface Area (m2 /g)
Pore Diameter Dv (nm)
Pore Volume (m3 /g)
ZnO-MCM-3 Reusable ZnO-MCM-3
739.543 420.100
2.180 3.043
0.547 0.233
Table 6. Binding energy and atomic of ZnO-MCM-3 after six photodegradation cycles of MO. BE/eV
Material ZnO-MCM-3 Reusable ZnO-MCM-3
Atomic/%
Zn 2p
N 1s
Si 2p
O 1s
1022.27 1022.37
-
102.84 102.92
532.19 531.89
Zn 2p 2.03 0.63
4. Conclusions In this study, ZnO-MCM-3 and ZnO-MCM-9 with photodegradation properties were prepared by two steps. Zinc ions were introduced to MCM-41 by coordinating with salicylaldimine modified on mesoporous silica and then a high-temperature oxidation was undertaken to transform zinc ions into zinc oxide and remove the salicylaldimine. The characterization confirmed that during the wet impregnation, the coordination of salicylaldimine could help to adsorb more zinc ions so that more ZnO would load on mesoporous silica. The more salicylaldimine was grafted on MCM-41, the more zinc ions would be caught and more nano zinc oxide would form. The appearance of hierarchical tunnels proved that ZnO were loaded in the channels of MCM-41. Furthermore, a suitable grafted amount of salicylaldimine as well as a good degree of orderliness would help MCM-41 to make those nano ZnO loaded in the channels grow into wurtzite crystalline. However, due to the lack of restrictions, nano ZnO loaded on the outside surface of MCM-41 were formed in spherical instead of wurtzite crystalline. The addition of ZnO-MCM did enhance the degradation efficiency of MO under UV radiation. The degradation of MO was mainly controlled by two physical constants. The surface areas of the ZnO-MCM played a dominant role in the first process while the loading amount of nano ZnO showed a more important role during the second process. Meanwhile, though the blocking effect of nano ZnO in the channels would reduce the orderliness of the vehicle, it was seen that this effect did not have much influence on the degradation efficiency. The pseudo-first-order model could be used to explain the photodegradation process. In brief, with the help of the coordination effect of the modified mesoporous silica, more metal ions will be adsorbed, so that more nano metal oxide will load on the mesoporous silica after the oxidization process. This method will make the mesoporous materials play a more significant role in the loading of nano metal oxide. Author Contributions: Z.S. and X.Z. conceived and designed the experiments; Z.S. performed the experiment; Z.S.; H.Z. and X.Z. analyzed the data; H.X. and H.C. contributed the reagents/materials; C.F. offered analysis tools; Z.S.; H.Z. and X.Z. wrote/edited the paper. Funding: This research was funded by National Natural Science Foundation of China (Grant No. 21576303, 21606262), Natural Science Foundation of Guangdong Province (Grant No. 2017A030311003), Science and Technology Program of Guangzhou, China (Grant No. 201604020054). Conflicts of Interest: The authors declare no competing financial interest.
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