Article pubs.acs.org/JPCC
Characterizing the Structure and Defect Concentration of ZnO Nanoparticles in a Colloidal Solution T. Rossi,† T. J. Penfold,‡ M. H. Rittmann-Frank,† M. Reinhard,† J. Rittmann,† C. N. Borca,§ D. Grolimund,§ C. J. Milne,‡ and M. Chergui*,† Laboratoire de Spectroscopie Ultrarapide, ISIC, École Polytechnique Fédérale de Lausanne (EPFL), FSB, Station 6, CH-1015 Lausanne, Switzerland ‡ SwissFEL and §Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland †
ABSTRACT: The structure and defect concentration of colloidal solutions of ZnO nanoparticles, synthesized by a sol−gel procedure (SG-NP), as well as commercially available ZnO nanoparticles (SA-NP) are investigated by UV−vis absorption spectroscopy, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). The XRD patterns, in agreement with the TEM images, reveal that the SGNPs are more ellipsoidal than the SA-NPs. XRD and XAS reveal the presence of both zinc and oxygen vacancies. The concentration of the latter is independent of the NP size. This is not the case for the zinc vacancies, whose concentration increases sharply in the SG-NPs compared to the SA-NPs, and an ∼40% oxygen excess in comparison to the expected stoichiometric ratio is found. Importantly, an extended X-ray absorption fine structure (EXAFS) analysis shows that this large concentration of zinc vacancies does not lead to distortions of the local lattice structure. Finally, the Zn K-edge X-ray absorption near edge structure (XANES) spectra show distinct changes in the rising edge and above edge regions, which supports the presence of zinc vacancies. In all cases, two weak pre-edge features are also observed and assigned to a small concentration of oxygen vacancies.
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INTRODUCTION Due to their remarkable physical properties, metal oxide nanostructures are a central focus for chemistry, physics, and materials science research.1,2 Among the most popular of these materials is zinc oxide (ZnO).3 This direct wide band gap (3.36 eV) semiconductor has received significant attention, owing to its potential applications in ultraviolet (UV) photodetectors,4,5 photocatalysis,6,7 varistors,8 solar cells,9−13 light-emitting diodes,14−16 or ion insertion batteries.17 In addition, as Zn is a d10 metal, it is attractive as an alloying component because it is unlikely to introduce gap states that promote electron−hole recombination.18,19 For solar energy conversion and photocatalysis, despite a better photocatalytic efficiency than TiO2 in some cases, the usability of ZnO in aqueous solution has been limited by instability arising from photocorrosion under UV irradiation.20−22 Importantly, for these cases, both the photoactivity and stability of the ZnO nanostructures are thought to be strongly affected by their morphology and defect concentration.23 Consequently, while ZnO remains a strong alternative to TiO2, due to its ease of crystallization and the possibility to synthesize a wide variety of nanostructures,12,24−26 for applications it is important to understand the structural and defect-related properties of ZnO. In pursuit of this, a large number of the studies have used photoluminescence to probe the defect induced optical properties.27−33 Extensive simulations have also been used to obtain energetic insight into the defects sites.3,34−38 These studies have revealed that, although a wide array of defect types © 2014 American Chemical Society
exists, the most common are oxygen and zinc vacancies. However, obtaining a full description of the morphology and defect concentration within these nanostructures requires structure-sensitive techniques such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS), and therefore it is rather surprising that the number of such investigations is small. Indeed, most previous XRD measurements have focused on characterizing the size and lattice parameters of ZnO nanostructures under a variety of different conditions, such as temperature and pressure.3 Using a size-broadening fit, Popa and Balzar39 demonstrated that it is also possible to extract the shape, orientation, and size distributions of crystallites in a ZnO powder. From a spectroscopic perspective, Hsu et al.40 used Zn Kedge XAS to study the formation of oxygen vacancies in Codoped ZnO samples. With the help of simulations they reported that oxygen vacancies give rise to significant variations in the spectrum, and in particular a distinct new transition in the pre-edge region at ∼9661 eV. Later, Jeong et al.41,42 used extended X-ray absorption fine structure (EXAFS) to probe the local geometry of three samples: two nanoparticles (NPs) of radius = 5 and 70 nm and a nanorod (radius = 40 nm, length = 0.5 μm). By fitting their spectra, they concluded that the bond lengths in the ZnO nanorods were slightly elongated along the axis of the rod, compared with ZnO powder, but did not Received: June 5, 2014 Revised: July 22, 2014 Published: July 29, 2014 19422
dx.doi.org/10.1021/jp505559u | J. Phys. Chem. C 2014, 118, 19422−19430
The Journal of Physical Chemistry C
Article
observe any defects. In a subsequent study,42 they reported a distorted tetrahedral coordination for the larger NP with one Zn−O bond length of 1.93 Å and the other three being 1.98 Å. For the smaller NP, they found a tetrahedral geometry, which was accompanied by a significant reduction in coordination, corresponding to 30% vacancies at both oxygen and zinc sites. In contrast, no vacancies were observed for the 70 nm NPs. Because ∼40% of the quantum dot sample is expected to be surface, the authors concluded that these vacancies occur at the surface. However, this large change in the concentration of oxygen vacancies as a function of the size of the NP is not consistent with the conclusions of Xiong et al.43 who used IR and Raman spectroscopy. Although these techniques are not directly structure-sensitive, they found that the spectral features assigned to oxygen vacancies did not vary with size. Besides the concentration of defects, their proximity also plays a crucial role in applications such as photocatalysis. By studying two defect-related photoluminescence bands at ∼500 and 550 nm, Ghosh et al.44 concluded that ZnO nanostructures are composed of a crystalline core and a defect-rich surface shell, which is similar to what is reported for TiO2.45,46 By comparing the relative intensities of the two emission bands as a function of particle size, they reported that for NPs with a radius of ≤20 nm, this defect rich surface has a radius of ∼4 nm. These conclusions were later used to interpret the nanosecond recombination dynamics of photoexcited bare and dye-sensitized ZnO NPs.48 Consequently, despite the large number of investigations into ZnO nanostructures, a quantitative description of the concentration of defects and the local geometrical distortions that they induce is still lacking. In this paper we use UV−vis absorption, transmission electron microscopy (TEM), XAS, and XRD to characterize the structure and defect concentration of two ZnO nanostructures in colloidal solution. The first, a ZnO NP synthesized using a sol−gel procedure (SG-NP), is in ethanol, while the second, a commercially available ZnO NP (SA-NP) is in either water or ethanol. Analysis of the XRD patterns reveals that the SG-NPs are more ellipsoidal, exhibiting a larger growth along one basal axis. Using both XRD and XAS, we identify and quantify the zinc and oxygen vacancies within the nanostructures. We find that the concentration of the latter is small and largely independent of the NP size. However, the concentration of zinc vacancies significantly increases in the smaller SG-NPs, for which a ∼40% oxygen excess in comparison to the expected stoichiometric ratio is identified. Using EXAFS we demonstrate that this large concentration of zinc vacancies does not lead to significant distortions of the local lattice structure. Finally, the Zn K-edge spectra show distinct changes in the rising edge and above edge regions, which support the presence of zinc vacancies. In all cases, two weak features are observed at the edge, which we assign to the presence of oxygen vacancies. These do not change as a function of size, in agreement with the EXAFS and XRD analysis.
The sol−gel (SG-NP) sample was synthesized following the procedure of Spanhel et al.,49 which we briefly describe hereafter. A mixture of absolute ethanol (0.5 L) and Zn(OAc)2· 2H2O (10.98 g) was distilled under ambient atmospheric pressure while avoiding exposure to moisture. The condensate was collected using a 1 L flask, Condensator, condensate receiver, and calcium chloride trap (CaCl2). Initially zinc acetate is only slightly soluble in ethanol but becomes totally soluble in boiling ethanol. Over a period of approximately 180 min, the solution was refluxed at 80 °C and vigorously stirred. The precipitate was filtered off and placed into an Erlenmeyer flask and diluted with 200 mL of absolute ethanol. Next, 100 mL of a 0.14 M LiOH·H2O solution in ethanol was made at 0 °C. An ultrasonic bath was used to fully dissolve the powder. This basic solution was added to the ethanolic solution dropwise under vigorous stirring at 0 °C. The hydrolysis was then prolonged for 30 min at 10 °C. The obtained solution is colorless and slightly white. The NPs were then precipitated to wash from them unreacted ZnAc and LiAc. Hexane was added to fractions of the solution in 50 mL plastic flasks in a ratio between 1:1 and 2:1 of hexane/ZnO to precipitate the quantum dots. The mixtures were centrifuged at 5000 rpm for 2 min. The supernatants were removed, and the solid precipitate was concentrated, washed with cold ethanol, and finally redispersed in ethanol with the help of an ultrasonic bath. Sample Characterization. The three ZnO samples were characterized by UV−vis absorption spectroscopy, TEM, XAS, and XRD. The UV−vis absorption measurements were conducted on a Perkin-Elmer Lambda 35 UV/vis spectrometer. The morphologies and dimensions of the samples were probed by TEM on a Jeol 2010 Microscope. Samples were deposited on a 400 copper grid with a carbon film. For the samples in ethanol, this was performed by evaporating the solvent at ambient temperature, while the samples in water were assisted by a plasma heater. The powder XRD measurements were carried out on a Bruker D8 Advance type diffractometer using a copper photocathode (λ1 = 1.54060 Å, λ2 = 1.54439 Å, Iλ1/Iλ2 = 0.5). The samples in ethanol were dried at ambient temperature, while those in water were slightly heated (40 °C). The Zn Kedge XAS measurements were conducted at the microXAS beamline of the Swiss Light Source synchrotron (Paul Scherrer Institut, Villigen, Switzerland). The sample was flowed through a closed-loop flow-capillary setup. The X-ray energy was calibrated by measuring a reference zinc foil. Both X-ray transmission (IT) and fluorescence (IF) signals were measured at the same time. A single-element silicon drift detector (Ketek AXAS-SDD10-138500, 10 mm2) with an energy resolution of ∼150 eV (ΔE/E ∼ 1.5%) was used for fluorescence detection (IF). It was placed at an angle normal to the incoming X-ray beam in order to minimize elastic scattering. The transmission signal was detected by a silicon diode (100 μm thick, integrated receiver/decoder (IRD)). The sample sizes were 25 mM, and 35 mM for ZnO NPs in ethanol and water, respectively, and 32 mM for the ZnO SG-NP in ethanol.
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EXPERIMENTAL SECTION Sample Preparation. The zinc oxide SA-NP samples were purchased from Sigma−Aldrich as colloidal dispersions. This sample was synthesized by the hydrolysis of a zinc salt in a polyol medium and heating at 160 °C according to ref 47. Using dynamic light scattering (DLS), both sample dispersed in either water (721077 Aldrich) or ethanol (721085 Aldrich) were found to have an average particle size
