Article pubs.acs.org/Langmuir
Microstructure-Controlled Aerosol−Gel Synthesis of ZnO Quantum Dots Dispersed in SiO2 Nanospheres Dudi Adi Firmansyah, Sang-Gyu Kim, Kwang-Sung Lee, Riyan Zahaf, Yong Ho Kim, and Donggeun Lee* School of Mechanical Engineering, Pusan Clean Coal Center, RIMT, Pusan National University, Busan 609-735, South Korea ABSTRACT: ZnO quantum dots dispersed in a silica matrix were synthesized from a TEOS:Zn(NO3)2 solution by a onestep aerosol−gel method. It was demonstrated that the molar concentration ratio of Zn to Si (Zn/Si) in the aqueous solution was an efficient parameter with which to control the size, the degree of agglomeration, and the microstructure of ZnO quantum dots (QDs) in the SiO2 matrix. When Zn/Si ≤ 0.5, unaggregated quantum dots as small as 2 nm were distributed preferentially inside SiO2 spheres. When Zn/Si ≥ 1.0, however, ZnO QDs of ∼7 nm were agglomerated and reached the SiO2 surface. When decreasing the ratio of the Zn/ Si, a blue shift in the band gap of ZnO was observed from the UV/Visible absorption spectra, representing the quantum size effect. The photoluminescence emission spectra at room temperature denoted two wide peaks of deep-level defect-related emissions at 2.2−2.8 eV. When decreasing Zn/Si, the first peak at ∼2.3 eV was blue-shifted in keeping with the decrease in the size of the QDs. Interestingly, the second visible peak at 2.8 eV disappeared in the surface-exposed ZnO QDs when Zn/Si ≥ 1.0. transparent SiO2 matrix through a sol−gel reaction11 or through a flame spray pyrolysis.12 In both cases, the size of the ZnO nanoparticles is controllable by varying the content of the SiO2 particles11 or the Si precursor.12 The thermal stability of the ZnO nanoparticles is greatly enhanced with a high content of SiO2. However, a majority of earlier studies showed that ZnO/SiO2 particles were randomly agglomerated. It is also very rare to find a controlled microstructure of ZnO QDs in a SiO2 matrix. Therefore, it remains a challenge to develop a new method to synthesize antiaging ZnO QDs in a manner that allows control of their size, agglomeration, and microstructure. After a careful comparison of different studies,11,12 we noticed that ZnO and SiO2 particles have been prepared before mixing or were coprecipitated in a flame. Thus, the violent interparticle (and interspecies) coagulations were unavoidable. This led to the idea that, upon the generation of SiO2 particles, ZnO QDs could be made to form inside the controlled SiO2 microstructure.13 This idea is realized in this study through a combination of an aerosol−gel reaction of a Si precursor and spray pyrolysis of a Zn precursor.
1. INTRODUCTION Zinc oxide (ZnO), one of the most important wide-band gap materials (3.37 eV, at 300 K), has a large exciton binding energy of 60 meV, resulting in efficient near-band-edge ultraviolet (UV) excitonic emission at ∼370 nm at room temperature. ZnO also features an excellent absorption property of UV light.1,2 In particular, ZnO nanoparticles, also known as quantum dots (QDs) when the particle radius is reduced to or less than the excitonic Bohr radius (∼2 nm) of bulk ZnO, are known to show remarkable blue shifts in their optical properties (UV light absorption and UV-to-visible light emission) due to the quantum confinement effect.3 The UV emission was generally attributed to the radiative recombination of excitons.2,4,5 On the other hand, ZnO nanoparticles often experience oxygen vacancies, mostly at the particle surface, leading to a significant orange-green6 or blue-green emission.7 The visible emission of the ZnO was attributed to the radiative recombination of electrons with holes trapped in surface-defect states (mainly by oxygen-vacancy defects).7 As a photocatalyst, surface defects of the ZnO nanoparticles are desired for the efficient use of visible light. For the proper use of UV lasing materials, these defects need to be healed by sintering at 200−500 °C in air in an effort to reduce the intensity of the visible luminescence.8 However, Carnes and Klabunde9 reported that ZnO QDs could grow by ∼1.4 times during calcination at 400 °C, degrading their optical properties. In addition, colloidal ZnO QDs created by wet chemistry often age (agglomerate) to a larger size over several days or even several hundred minutes, leading to a significant red shift in their absorption spectra as well as photoluminescence peaks.10,11 An alternative method to prevent the growth of ZnO QDs during sintering is to embed them in a © 2012 American Chemical Society
2. EXPERIMENTAL SECTION ZnO QDs with a controlled size in SiO2 matrices are generated through a one-step aerosol−gel method, as schematically shown in Figure 1. Tetraethoxysilane (TEOS, Si(OCH2CH3)4) is dissolved in a mixture of deionized water, hydrochloric acid (HCl), and ethanol, followed by the addition of zinc nitrate (Zn(NO3)2·6H2O) under Received: September 22, 2011 Revised: December 22, 2011 Published: January 5, 2012 2890
dx.doi.org/10.1021/la203730a | Langmuir 2012, 28, 2890−2896
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Article
(eq 1), alcohol condensation (eq 2), and water condensation (eq 3).15
Si−OR + H2O → Si−OH + ROH
(1)
Si−OR + OH−Si → Si−O−Si + ROH
(2)
Si−OH + OH−Si → Si−O−Si + H2O
(3)
When HCl is added as the acid catalyst, the hydrolysis reaction (eq 1) is enhanced and produces a larger number of Si−OH molecules. Thus, the water condensation reaction (eq 3) likely dominated the SiO2 sol formation. This activates violent aggregation of the primary sol particles, leading to a fine network of silica structures. Therefore, the pH of the solution inside the atomizer is maintained at 3.0. As noted in section 2, SiO2 sols continue to grow to large aggregates as the reaction time increases. By means of dynamic light scattering,16,17 the hydrodynamic size of the silica aggregates was found to increase proportionally with the time from 100 nm (4 h) to 270 nm (12 h). This transient growth of silica particles is attributed to the low pH of the solution (3.0), close to the point of zero charge of SiO2, at which the silica colloid becomes unstable.16−18 According to Rubio et al.,19 primary particles become smaller as the H2O/TEOS ratio increases. It is postulated that smaller primary particles build up denser networks with smaller void spaces in which Zn(NO3)2 mainly decomposes. Thus, the final size of the ZnO QDs may be minimized when denser networks are produced. This is why such a high ratio of H2O/TEOS (231:1) is currently used. TEM images indeed confirmed that the primary size of SiO2 particles were smaller than 10 nm.13 Figure 2 shows XRD patterns of ZnO/SiO2 particles obtained at three different furnace temperatures of 300, 500,
Figure 1. Experimental setup of aerosol−gel synthesis of ZnO quantum dots in SiO2 matrix. continuous stirring of the mixture at 60 °C. The molar ratio of these substances in the mixture (TEOS/H2O/EtOH/Zn(NO3)2) is kept at 1:231:68:1 as a standard condition. The molar ratio of TEOS to Zn(NO3)2 is then varied to 1:0.1, 1:0.5, 1:1, and 1:2, corresponding to the change of the mole fraction of Zn, i.e., Zn/(Zn + Si) from 0.09 to 0.67. The TEOS continues to be sol−gel reacted until the solution is atomized, but Zn(NO3)2 is kept dissolved (unreacted) in the solution. This enables the kinetic separation of the productions of the SiO2 and ZnO particles. Dynamic light scattering (DLS, ELS-8000, Otsuka, Japan) was used for the in situ measurement of the SiO2 agglomerates in the solution through the measurement of the diffusion coefficient of the particles. The aqueous solution is atomized with high-pressure air (30 psi), forming 1−5 μm droplets. The droplets are partially dried in a diffusion dryer filled with dried silica gel and then passed to a 30 cm long tube furnace. The time from the start of the stirring to the beginning of the atomization stage is termed the reaction time. The reaction time varies from 4 to 12 h to control the size of the SiO2 colloidal aggregates. When a droplet is heated and the solvent evaporates in the furnace, SiO2 sols (or small aggregates) agglomerate more to form a network and then begin to sinter. During the shrinkage of the droplet, the coarse silica network becomes denser due to the additional sol−gel reaction of the TEOS, very likely accompanied by the decomposition of Zn(NO3)2. Thermogravimetric analysis (Thermogravimetric Analyzer, TGA, Q50) was conducted on the Zn(NO3)2 powder in air while increasing the temperature to 300 °C at a rate of 4 °C/min. The results confirmed the complete pyrolysis of Zn(NO3)2 at 300 °C. Hence the furnace temperature varied from 300 to 600 °C. In this process, the air flow rate is kept at 1 lpm to keep the droplet residence time in the furnace at ∼1 s, thereby providing sufficient time for evaporation and pyrolysis. The final product particles collected on a filter are used for further characterization. More details are available in the literature.13,14 A scanning mobility particle sizer (SMPS) was employed to measure the size distribution of the aerosol particles. X-ray diffraction (XRD, D/max 2400, Rigaku) was used to measure the size and crystalline phase of the ZnO QDs. Powder samples collected in the filter were also dispersed in ethanol and then dropped onto a carboncoated TEM grid. The particle size, morphology, crystallinity, and elemental composition of the ZnO QDs dispersed in the SiO2 matrices were investigated using a high-resolution transmission electron microscope (HRTEM, Hitachi H-7600, 200 keV) equipped with energy dispersive spectroscopy (EDS). The particle surface composition is examined by X-ray photoelectron spectroscopy with an Mg Kα source (XPS, ESCALAB 250, VG Scientifics). The absorption properties and photoluminescence characteristics of the ZnO QDs were measured here using UV/Visible absorption spectroscopy (SD 100 uv−vis spectrophotometer, Scinco, Korea) and with a benchtop fluorometer that utilizes a Xenon lamp at 350 nm (QuantaMaster 30, PTI), respectively.
Figure 2. X-ray diffraction patterns of as-received SiO2−ZnO powders at different reactor temperatures. The various peaks of the sample obtained at 300 °C are identified to Zn nitrate and two kinds of intermediate species as indexed by (1) Zn(H2O)6(NO3)2 (JCPDS file no. 720058), (2) Zn5(OH)8(NO3)2(H2O)2 (no. 720627), and (3) Zn(OH)(NO3)(H2O) (no. 841907).
and 600 °C. Though a dynamic TGA indicates that 300 °C is high enough for the complete decomposition of Zn(NO3)2, the residence time in the furnace is much shorter than that in a TGA machine. As seen in Figure 2, there remains a mixture of unreacted Zn nitrate and two kinds of intermediate species at 300 °C, indicating an incomplete decomposition of Zn nitrate. On the other hand, the decomposition reaction appears to be complete at 600 °C, as only three major peaks3 of the (100), (002), and (101) crystalline facets of ZnO are observed. The peak analysis reveals that the ZnO QDs have a hexagonal
3. RESULTS AND DISCUSSION 3.1. Determination of the Experimental Parameters. The sol−gel reaction consists of three main stages: hydrolysis 2891
dx.doi.org/10.1021/la203730a | Langmuir 2012, 28, 2890−2896
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wurtzite structure with lattice parameters, a and c, of 3.221 and 5.259 Å, respectively. It is noted that the lattice parameters are very close to those of bulk ZnO indicated by JCPDS file no. 751526, suggesting that the ZnO QDs are in zero strain state. Also, the absence of crystalline SiO2 peaks indicates that the SiO2 species are amorphous. The furnace temperature was fixed at 600 °C for further experiments. To investigate the effect of the reaction time, HR-TEM images of the as-received particles obtained at 4 and 12 h are compared in Figure 3. As denoted by the solid circles, many
QDs, the concentration ratio of TEOS to Zn(NO3)2 is varied from 1:0.1 to 1:2. The HRTEM images taken at each ratio are shown in Figure 4. The inset of Figure 4 shows the overall morphology of a single SiO2 sphere embedding many darker ZnO QDs. A part of the SiO2 sphere (circled in the inset) is magnified and shown in Figure 4a−d. At a ratio of 1:0.1 (the minimum concentration of Zn(NO3)2), isolated ZnO QDs with a size of 1−3 nm (count mean diameter = 2 nm) are well dispersed in the amorphous silica matrix, as highlighted by the dotted circles in Figure 4a. It is clear that the ZnO QDs are well crystallized. When the content of Zn(NO3)2 increases to 1:0.5 and 1:2, the ZnO QDs grow to 3 and 7 nm, respectively. In addition, the ZnO particles appear to be overlapped or agglomerated above a concentration ratio of 1:1. Crystallite sizes of the ZnO QDs were calculated from the XRD profiles by the Scherrer equation. At the ratios of 1:1 and 1:2, the crystallite sizes are found to 5.6 and 6.5 nm, respectively, which are fairly consistent with the sizes obtained from the HR-TEM. On the other hand, when the content of Zn becomes 1:0.5 or less, three tiny humps were observed in XRD pattern (not shown here), implying that the ZnO QDs is so small. Of particular interest is that the ZnO QDs exist only in the inner region of the sphere at a low concentration of Zn(NO3)2 (
