Two-Solvent Method Synthesis of NiO/ZnO Nanoparticles Embedded in Mesoporous SBA-15: Photocatalytic Properties Study
© Dai et al. 2016
Received: 20 January 2016
Accepted: 19 April 2016
Published: 27 April 2016
Different loadings of NiO/ZnO nanoparticles embedded in mesoporous silica (SBA-15) were prepared via a two-solvent method with the ordered hexagonal mesoporous structure of SBA-15 kept. X-ray diffraction, transmission electron microscope, X-ray photoelectron spectroscopy, diffusive reflective UV-vis spectroscopy, and N2 adsorption porosimetry were employed to characterize the nanocomposites. The results indicate that the ordered hexagonal mesoporous structure of SBA-15 is kept and the absorption band edges of the nanocomposites shift into the ultraviolet light regime. The photocatalytic activity of our samples for degradation of methylene orange was investigated under UV light irradiation, and the results show that the nanocomposites have higher photodegradation ability toward methylene orange than commercial pure P-25. The photocatalytic activity of the nanocomposites was found to be dependent on both the adsorption ability of the SBA-15 and the photocatalytic activity of NiO-ZnO nanoparticles encapsulated in SBA-15. In addition, there is an optimal loading of NiO-ZnO nanoparticles. Too high or low loading will lower the photodegradation ability of the nanocomposites.
Over the past two decades, semiconductor metal oxides have become the promising photocatalysts in environmental remediation for their nontoxic nature, low cost, and chemical stability [1–3]. Among these semiconductor metal oxides, zinc oxide (ZnO) has been recognized as an excellent material for photocatalytic processes for its high photosensitivity, low cost, and environmental friendliness [4–6]. However, the commercial application of ZnO is still subject to higher band gap (about 3.37 eV) and faster recombination of photogenerated electron-hole pairs. To solve these problems, great efforts have been made to enhance the photocatalytic performance of ZnO nanostructures. Recently, the preparation of ZnO-based heterojunctions including noble metal, and other selected semiconductor with regulated bands, is reported for the enhancement of photocatalytic performance, which can facilitate the mutual transfer of photogenerated electrons and holes and help to suppress the recombination of electron and hole [7, 8]. Among these heterojunctions, NiO-ZnO heterojunctural nanostructures have received special research interest. NiO, a p-type semiconductor (Eg = 3.5 eV) possesses high hole concentration, high hole mobility, and low lattice mismatch with ZnO, which facilitates the formation of p-n heterojunction with ZnO [9, 10]. Recently, electrospun and thermal oxidization methods have been reported for the preparation of NiO-ZnO heterojunctions with enhanced photocatalytic activities [11, 12]. However, these methods are subject either to high cost equipment or nanoparticle aggregation, which increases the grain size, reduces the specific surface area, and depresses the photocatalytic activity. Therefore, the exploitation of a new way for the facile preparation of NiO-ZnO heterojunctions is still highly desired.
In this paper, a facile impregnation technique named “two-solvent” method was developed for the preparation of NiO-ZnO nanoparticles encapsulated in mesoporous SBA-15 silica. Hydrophobic alkane solvent (n-hexane, n-pentane, or cyclohexane) were first used to impregnate SBA-15 [13–15], increasing the number of germinal and hydrogen bonded silanol, which can result in better wettability and facilitate the introduction of aqueous solutions into the pores. When metal precursor aqueous solution is introduced, the droplets of the solution are liable to be pushed into the pore channels of SBA-15 for the nonpolar hydrophobic environment and the hydrophilic nature of SBA-15. Thus, guest species can be confined and homogeneously distributed within the pores of SBA-15, which prevents nanoparticle aggregation. To our best knowledge, there has been no report on the preparation of NiO/ZnO nanoparticles encapsulated in SBA-15.
Synthesis of NiO/ZnO/SBA-15 Nanocomposites
All chemicals in this paper are analytical pure grade and used as received. The mesoporous silica SBA-15 was synthesized based on ref . The NiO/ZnO/SBA-15 nanocomposites were fabricated by the two-solvent impregnation method at 298 K . In a typical synthesis, 1 g of extracted mesoporous silica SBA-15 was added into 30 mL of n-hexane (hydrophobic solvent) with vigorous stirring of 2 h. Then, 0.98 mL of the mixture solution (hydrophilic solvent) of Zn(NO3)2·6H2O and Ni(NO3)2·6H2O with the atom ratio of Zn to Ni of 1:1 were introduced to the above mixture dropwise with vigorous stirring until a paste-like product turned up, which was dried for 12 h ambiently. Finally, NiO/ZnO/SBA-15 nanocomposites were obtained by calcining the dried product at 550 °C for 5 h in a muffle oven at a ramping rate of 1 °C/min. The NiO/ZnO/SBA-15 nanocomposites with different loading of NiO/ZnO nanoparticles are labeled as x % NiO/ZnO/SBA-15, where x shows the weight percentage of NiO and ZnO in the nanocomposites (the atom ratio of Zn to Ni is kept to be 1:1). Moreover, 30 % NiO/SBA-15 and 30 % ZnO/SBA-15 were also prepared for the comparative study of photocatalytic properties via the same procedure.
Characterization and Measurements
X-ray diffraction (XRD) patterns were collected on an 18 kW advanced X-ray diffractometer equipped with Cu Kα radiation (λ = 1.54056 Å). Transmission electron microscopy (TEM) patterns were obtained with JEOL JEM-2100, operating at 200 kV. Brunauer-Emmett-Teller (BET) surface and the pore structures of the samples were measured at 77 K by a micromeritics ASAP2010 nitrogen isothermal adsorption instrument. The pore volumes were determined at a P/P 0 value of 0.97, the specific surface areas were computed with the BET equation, and the mean pore diameters were calculated with the Barrett-Joyner-Halenda (BJH) method based on the adsorption branch of the N2 isotherm curve. X-ray photoelectron spectroscopy (XPS) was performed on a PHI-5702 instrument with Al Kα radiation as the excitation source (hν = 1486.7 eV). UV-vis absorption spectra were recorded on a shimadzu 240 UV-vis spectrophotometer.
Photocatalytic Experimental Details
The photocatalytic degradation experiments for methylene orange (MO) were carried out in a self-prepared reactor. In the degradation procedure, samples were immersed in a 50-mL beaker containing 40 mL of MO aqueous solution (20 mg/L). Before the solution was irradiated by a 350 W Xenon lamp, the vertical distance between the solution level and the horizontal plane of the lamp was fixed at 10 cm. At an interval of 10 min, 3 mL of solution was taken out from the reactor. The absorbance of the solution was determined on a UV-vis absorption photometer (UV-3200S, MAPADA analytic apparatus Ltd. Inc., Shanghai, China) at a 464-nm wavelength.
Results and Discussion
Characteristic of NiO/ZnO/SBA-15 Nanocomposites
Physicochemical parameters derived from nitrogen physisorption and XRD data for different samples (d 100 is the interplanar spacing of the hexagonal structure; a 0 represents the pore-to-pore distance of the hexagonal structure)
d 100 (nm)
a 0 (nm)
S BET (m2/g)
Pore diameter (nm)
Pore volume (cm3/g)
Photocatalytic Measurements on NiO/ZnO/SBA-15 Nanocomposites
Compared with sample 20 % NiO/ZnO/SBA-15 (k = 0.0484 min−1) and 40 % NiO/ZnO/SBA-15 (k = 0.0518 min−1), sample 30 % NiO/ZnO/SBA-15 shows the highest photocatalytic activity (k = 0.0588 min−1), suggesting that there is an optimal loading dosage of NiO/ZnO nanoparticles. In fact, with the decrease of NiO/ZnO loading, the adsorption of NiO/ZnO/SBA-15 nanocomposites becomes increased and the size of NiO/ZnO nanoparticles is decreased. As well known, a higher adsorption is beneficial to the improvement of photoactivity, while smaller nanoparticles in the pore channels of SBA-15 will increase the distance between photoactive sites and the adsorption sites at the surface and enhance the recombination rate of photoinduced electron-hole pairs.
In summary, NiO/ZnO/SBA-15 nanocomposites with different NiO/ZnO loading were successfully fabricated by using a two-solvent method. The photocatalytic measurement results indicated that the as-obtained NiO/ZnO/SBA-15 nanocomposites possessed higher photocatalytic activity than Degussa P-25 for the degradation of MO dye under UV light irradiation. Higher adsorption performance and enhanced separation efficiency of photogenerated electron-hole pairs are believed to be responsible for the great enhancement of photocatalytic activity. Moreover, there exists an optimal loading of NiO/ZnO for NiO/ZnO/SBA-15 nanocomposites in terms of photocatalytic activity, which is dependent of the adsorption of nanocomposites and NiO/ZnO nanoparticles sizes.
This work was financed by the 211 project of Anhui University, National Natural Science Foundation of China (11374013, 51502002, 11404001, 61290301), Provincial Science Foundation of Anhui (1608085ME97), Outstanding young talent fund of Anhui Province (J05201424), and Research Fund for the Doctoral Program of Higher Education of China (20133401110002).
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