Structure and Photoluminescent Properties of ZnO Encapsulated in Mesoporous Silica SBA-15 Fabricated by Two-Solvent Strategy
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The two-solvent method was employed to prepare ZnO encapsulated in mesoporous silica (ZnO/SBA-15). The prepared ZnO/SBA-15 samples have been studied by X-ray diffraction, transmission electron microscope, X-ray photoelectron spectroscopy, nitrogen adsorption–desorption isotherm, and photoluminescence spectroscopy. The ZnO/SBA-15 nanocomposite has the ordered hexagonal mesostructure of SBA-15. ZnO clusters of a high loading are distributed in the channels of SBA-15. Photoluminescence spectra show the UV emission band around 368 nm, the violet emission around 420 nm, and the blue emission around 457 nm. The UV emission is attributed to band-edge emission of ZnO. The violet emission results from the oxygen vacancies on the ZnO–SiO2interface traps. The blue emission is from the oxygen vacancies or interstitial zinc ions of ZnO. The UV emission and blue emission show a blue-shift phenomenon due to quantum-confinement-induced energy gap enhancement of ZnO clusters. The ZnO clusters encapsulated in SBA-15 can be used as light-emitting diodes and ultraviolet nanolasers.
KeywordsZnO Clusters Mesoporous silica Photoluminescence
Semiconductors usually exhibit quantum size effects and electric and optical properties different from bulk materials, when their particle size decreases to nanometer scale . The fabrication strategy for semiconductor nanostructure includes a wide variety of vapor, liquid, and solid state processing routes . Different techniques such as pulsed laser deposition, sputtering, thermal evaporation and condensation, solid state reaction, and chemical method have been employed to fabricate such nanostructures. Among these techniques, the template-assisted synthesis, which involves confined growth of nanostructures because of volume space effect, provides a simple, low-cost, and high-yield synthetic route for a large variety of materials. Among various hard templates such as track-etched polycarbonate, polystyrene sphere (PS) colloidal monolayer template, single-walled carbon nanotube, and anodized aluminum oxide (AAO), ordered mesoporous silica SBA-15  is one prominent example and was used to construct nanostructures because of its uniform pore size, hexagonal array of one-dimensional cylindrical channels, large surface areas, and high thermal stability. Therefore, it is convenient to stabilize highly dispersed ultrafine metal or oxide nanocrystals, nanowires, quantum dots, and clusters in the channels of SBA-15.
ZnO is a multifunctional semiconductor material. Due to the features such as a wide band gap of 3.37 eV, a high exciton binding energy of 60 meV at room temperature, and special electrical and optoelectronic properties , a wide range of potential applications  from fine photoelectronics, transparent conductive films, solar cell windows, and acoustic wave devices to gas sensing devices excite intensive studies on ZnO nanostructures. In addition to conventional nanoparticles, the various ZnO nanostructures including quantum dots , nanowires , nanorods , and nanocastles  have been found to show unique optical, optoelectronic, and photocatalytic properties. The composite of carbon nanoparticles embedded in ZnO matrix was studied as a solar thermal absorber in solar energy applications . Furthermore, ZnO is a promising material for potential optical applications  and has potential application as a short-wave-length light emitting material. Now, a lot of studies are concentrated on tuning the band gap of ZnO by changing the diameter of particle size because of strong size-dependent band gap. All these have excited researches to develop new synthetic methodologies to prepare well-controlled ZnO nanostructures.
Up-to-date, several strategies have been developed to incorporate ZnO in the channels of mesoporous silica SBA-15 and MCM-41 and the pores of zeolites. Two examples are conventional wetness impregnation [12, 13, 14, 15] and the improved method with modification of the surface walls followed by loading precursor through affinity interaction [16, 17]. Generally, the former method seems difficult to completely avoid adsorptions of the ZnO precursor on the outer surface of the host template. The uncontrolled ZnO aggregation on the external surface of mesoporous silica will form in subsequent calcinations. The latter method involves complicated process and has low yield. So, it is necessary to develop a simple and low cost novel strategy to prepare ZnO encapsulated in SBA-15 with high quantum size effect and thermal stability.
Recently, a novel strategy called two-solvent method containing hydrophilic and hydrophobic solvents has been applied to prepare CoFe2O4 nanowires in carbon nanotubes [18, 19]. This method is based on a volume of precursor aqueous solution equal to the pore volume of host template materials which has the advantages of confining and distributing guest species within the pores of host template. Therefore, mesoporous silica SBA-15 could be regarded as a nanoreactor for constructing guest nanomaterials with controlled size and size distribution. The two-solvent method may be employed to prepare a nanocomposite of ZnO clusters supported in mesoporous silica. However, the synthesis of zinc oxide encapsulated in mesoporous silica SBA-15 by the two-solvent method has not been studied so far.
In our present work, we prepared a nanocomposite of ZnO clusters supported in mesoporous silica (or ZnO/SBA-15) via the two-solvent synthetic route. The structure and photoluminescent properties of the ZnO/SBA-15 nanocomposite were studied. The results show that ZnO clusters are distributed in the channels of SBA-15 without aggregations found on the external surface of SBA-15. The UV and blue emissions show a significant blue shift due to quantum size effect compared to the emission of the bulk counterpart reported in the literatures.
The ZnO/SBA-15 nanocomposite was prepared by incorporating zinc nitrate precursor into the channels of mesoporous silica SBA-15 and subsequent calcination. Parent mesoporous silica SBA-15 was synthesized according to the reported process . A typical synthetic procedure was carried out as follows: 4 g of triblock copolymer P123 [HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H, abbreviated as EO20PO70EO20, was mixed with 120 mL of 2 M hydrochloric acid (HCl) and 30 mL of deionized water. The mixture was stirred at 38 °C until P123 was completely dissolved. A total of 8.5 g of tetraethyl orthosilicate (TEOS) was added to this solution under vigorous stirring. The final mixture was stirred at 38 °C for 24 h, then transferred into a teflon-lined autoclave, and kept in the autoclave at 100 °C for 24 h under static condition for hydrothermal treatment. Finally, the formed white precipitates were filtered, washed with water, and dried at room temperature. The extracted mesoporous silica SBA-15 was obtained by removing P123 with ethanol extraction method under refluxing condition.
The procedure of incorporating ZnO into the channels of SBA-15 is as follows . A total of 1 g of extracted mesoporous silica SBA-15 was suspended in 20 mL of n-hexane as the first hydrophobic solvent; and the mixture was stirred for 2 h. A total of 0.98 mL of zinc nitrate solution of different concentrations as the second hydrophilic solvent was added to the above mixture dropwise. The resulting solution was vigorously stirred until a paste-like product was obtained. The paste-like product was dried for 12 h in air at room temperature. Finally, ZnO/SBA-15 nanocomposite was obtained by calcining the dried (paste-like) product at 500 °C for 4 h at a heating rate of 1 °C/min in air. The ZnO/SBA-15 nanocomposites with different ZnO loadings are referred as x wt% ZnO/SBA-15, where x represents the weight percentage of ZnO in the nanocomposite.
Low-angle and wide-angle X-ray diffraction (XRD) measurements were carried out on a Rigaku D/Max-2400 X-ray diffractometer using CuKαradiation in θ − 2θ scan mode. High resolution transmission electron microscope (HRTEM) observations and energy dispersive spectroscopy (EDS) measurements were conducted on a JEOL JEM 2010 electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI-5702 spectrometer (Physical Electronics, Inc.) using an AlKαX-ray source (1486.7 eV). The energy scale was internally calibrated by referencing to the binding energy of the C 1s peak of a carbon contaminant at 284.6 eV. Nitrogen adsorption–desorption isotherms were measured by a Micromeritics ASAP2010 system. Barrett–Emmett–Tellter (BET) method in the relative pressureP/P0range of 0.01–0.20 was applied for calculating specific surface areas. The pore volume was determined from the adsorption branch of the N2isotherm curve at theP/P0 = 0.97 signal point. The pore diameter was derived from the maximum of the pore size distribution curve obtained using Barrett–Joyner–Halenda (BJH) method based on the adsorption branch of the N2isotherm curve. Room temperature photoluminescence (PL) spectra were recorded on a FLS-920T fluorescence spectrophotometer with Xe 900 (450 W xenon arc lamp) as the light source using an excitation wavelength of 325 nm.
Results and Discussion
Physicochemical parameters derived from nitrogen physisorption and XRD datas for different samples (d100is the interplanar spacing of the hexagonal structure;a0represents the pore-to-pore distance of the hexagonal structure)
Pore diameter (nm)
Pore volume (cm3/g)
8 wt% ZnO/SBA-15
15 wt% ZnO/SBA-15
20 wt% ZnO/SBA-15
Combining the above low-angle XRD and TEM analysis results, the ZnO/SBA-15 nanocomposites have the ordered hexagonal mesostructures of SBA-15. The EDS analysis shows the presence of the Zn element in the ZnO/SBA-15 nanocomposites. The XPS analysis confirms that ZnO exists in the ZnO/SBA-15 nanocomposite. The wide-angle XRD shows that ZnO in the ZnO/SBA-15 nanocomposite calcined at 500 °C exists in the non-crystalline state. Nitrogen adsorption–desorption isotherms proves that ZnO exists in the channels of SBA-15. These observed results suggest that ZnO may exist as non-crystalline clusters in the channels of SBA-15, as the reported intrapore formation of guest species inside mesoporous silica [32, 33].
It is commonly known that ZnO exhibits the UV near-band-edge emission peak at around 380 nm and the visible emission band ranging from 440 to 600 nm . In general, the emission band in the visible region is associated with structural defects in ZnO . The UV near-band-edge emission peak is attributed to the recombination of free excitons  and depends on the ZnO particle size due to quantum size effect.
In our work, the emission band centered at about 370 nm can be fitted by three emission Gaussian peaks shown in the inset in Fig. 9; these three peaks are located at 368 nm, 420 nm, and 457 nm, respectively. Obviously, the UV emission peak centered at 368 nm should be ascribed to the radiative transition in electron-hole recombination process . Usually, ZnO exhibits the UV near-band-edge emission peak at around 380 nm. It is known that the band gap width increases as the particle size decreases, if the size of the ZnO particle decreases to the order of the Bohr radius. The band edge emission shows a blue shift . The blue shift of the UV near-band-edge emission of the ZnO/SBA-15 nanocomposite is observed due to a quantum-confinement-induced energy gap enhancement of the ZnO clusters, and the blue shift to short wavelength is much larger than those reported for mesoporous silica supported ZnO [12, 13, 15, 17]. The UV emission is broad. This may be correlated to defect states of the ZnO clusters, such as the bound exciton and the acceptor–donor pairs. This phenomenon is similar to the case of ZnO quantum dots . The violet luminescence centered at 420 nm is also observed. Recently, Shi et al. reported that the violet luminescence was observed from MCM-41 supported ZnO clusters, which is due to radiative transition between the interface traps and the valence band. The interface traps exist within the depletion regions at the ZnO–SiO2 boundaries . In our work, the mesoporous silica SBA-15 has a large surface areas about 806 m2/g determined by N2 adsorption. The ZnO–SiO2 interface traps exist in the ZnO/SBA-15 nanocomposites because of the distribution of the ultrafine ZnO clusters in SBA-15. The violet luminescence of the ZnO/SBA-15 nanocomposite centered at 420 nm should be attributed to the radiative transitions between the interface traps and the valence band. Similar results have also been reported in the ZnO/SBA-15 nanocomposite . The weak and broad blue emission band centered at 457 nm is a deep-level emission originated from the oxygen vacancies or interstitial zinc ions of ZnO . The blue shift to short wavelength was also observed, when compared to the value of 480 nm in the SBA-15 supported ZnO . It is found that the blue shift in the visible emission with decreasing particle size closely follows the blue shift in the band edge emission. This phenomenon is similar to the reported results for the ZnO quantum particle thin films . Due to its photoluminescent properties, the ZnO/SBA-15 nanocomposite has the potential applications as ultra-violet light-emitting diodes, laser diodes, and other optical devices. Besides, the ZnO/SBA-15 nanocomposite may be useful for detecting the nitrosamine content in solution, suggesting its potential applications in sensing carcinogens such as N′-nitrosonornicotine (NNN) in environment.
The two-solvent method was employed to prepare the nanocomposites of the ZnO clusters supported in SBA-15. The prepared ZnO/SBA-15 nanocomposites with high loadings of ZnO keeps well ordered hexagonal mesoporous structure of SBA-15. ZnO in the ZnO/SBA-15 nanocomposite calcined at 500 °C exists in form of non-crystalline clusters distributed in the channels of SBA-15. Room temperature photoluminescence spectra show three emission bands assigned to the UV band-edge emission (368 nm), the violet emission (420 nm), and the blue emission (457 nm). The blue shift of the UV band-edge emission and the blue emission for the ZnO/SBA-15 nanocomposites indicates that the ZnO clusters supported in SBA-15 have quantum size effect. The ZnO/SBA-15 nanocomposites have potential application as a short-wave-length light emitting material.
This work was supported by the International S&T Cooperation Program (ISCP) under 2008DFA50340, MOST, China and the National Natural Science Foundation of China under 50872046.