ARTICLE pubs.acs.org/JPCC
Few-Layer ZnO Nanosheets: Preparation, Properties, and Films with Exposed {001} Facets Jan Demel,† Josef Plestil,‡ Petr Bezdicka,† Pavel Janda,§ Mariana Klementova,† and Kamil Lang*,† ez 1001, 250 68 R ez, Czech Republic Institute of Inorganic Chemistry of the AS CR, v.v.i., Husinec-R Institute of Macromolecular Chemistry of the AS CR, v.v.i., Heyrovskeho nam. 2, 162 06 Praha, Czech Republic § J. Heyrovsky Institute of Physical Chemistry of the AS CR, v.v.i., Dolejskova 3, 182 23 Praha 8, Czech Republic † ‡
bS Supporting Information ABSTRACT: Highly crystalline ZnO nanosheets have been prepared by the solvothermal transformation of a delaminated layered zinc hydroxide salt. This strategy has proven to be reproducible and scalable. The nanosheets, with a total thickness of 0.6 0.7 nm, are built from two or three stacked ZnO tetrahedral layers. The lateral size varies between 15 and 25 nm. The nanosheets can be arranged into transparent films with a high area of polar {001} facets. Defects in the ZnO films can be tuned by annealing.
’ INTRODUCTION During the last decades, crystal morphology has been extensively studied because crystallographic faces differ in geometric and electronic structure and hence exhibit different physicochemical properties (e.g., in catalysis, optics, and luminescence).1,2 Enhanced properties are usually associated with high-surface-energy crystal faces. These surfaces can diminish rapidly during crystal growth as a result of the minimization of surface energy. Therefore, great effort has been devoted to the synthesis of micro-/nanoparticles terminated with high-energy surfaces.3 5 Among the metal oxides, zinc oxide plays a prominent role due to its luminescence, semiconducting, catalytic, and piezoelectric properties. The wurtzite-structured ZnO consists of alternating planes of tetrahedrally coordinated oxygen and zinc atoms arranged along the c axis. This anisotropy results in the spontaneous polarization of certain planes, e.g., (001). Other planes, such as (100), are apolar. During growth, ZnO nanostructures tend to maximize the exposed areas of the apolar planes because of their low surface energies and to minimize the areas of polar planes.6 The nanoparticles with large polar (001) surfaces receive special attention due to their enhanced catalytic7,8 and photocatalytic9,10 activity and their sensitivity as gas sensors.11,13 The morphology of the ZnO nanostructures can be controlled, and the procedures for the preparation of ZnO structures including spheres,12 flakes,13 rods,14 wires, belts,15 and hexagonal pyramids16 have been reported. Traditionally, platelike particles of ZnO are prepared using capping agents9 or by vapor deposition techniques;11,17 however, these methods lead to plate sizes on the micrometer scale. To prepare two-dimensional (2D) nanostructures of ZnO, an alternative approach is needed. One of the methods utilized for the preparation of nanometersized sheets is the delamination of layered inorganic materials.18 r 2011 American Chemical Society
The delamination of layered hydroxides (e.g., layered double hydroxides19,20 or layered single metal hydroxides21,22) produces brucite-like hydroxide nanosheets with a charge density controlled by the metal composition. These nanosheets can be transformed into metal oxides; however, higher temperatures are required.23 In our recent work, we have delaminated layered zinc hydroxide containing intercalated dodecyl sulfate (LZH-DS) in n-butanol (nBuOH) at 60 °C and have transformed it into ZnO nanoparticles.22 Herein, we report on the solvothermal transformation of the delaminated LZH-DS into few-layer nanosheets of ZnO that can be arranged into transparent films with exposed {001} facets dominant. The ZnO nanosheets, which are characterized by powder X-ray diffraction, high-resolution transmission electron microscopy, atomic force microscopy, and small-angle X-ray scattering, have sizes comparable to the original LZH hydroxide nanosheets, i.e., a lateral size of 15 25 nm and a thickness of 0.6 0.7 nm. The pred nanosheets are well-developed crystals that contain two or three stacked ZnO tetrahedral layers along the c axis, thus representing an analogue to few-layer graphene.
’ EXPERIMENTAL METHODS Materials. Sodium dodecyl sulfate (DS), n-hexanol (nHxOH), n-octanol (nOctOH) (all Aldrich), methanol (MeOH), ethanol (EtOH), i-propanol (iPrOH), n-butanol (nBuOH) (all Lachner, Neratovice, Czech Republic), Zn(NO 3 )2 3 6H 2 O, and NaOH, Received: October 17, 2011 Revised: November 10, 2011 Published: November 10, 2011 24702
dx.doi.org/10.1021/jp209973t | J. Phys. Chem. C 2011, 115, 24702–24706
The Journal of Physical Chemistry C (all Penta, Chrudim, Czech Republic) were used as purchased. Quartz plates (SPI supplies) were washed with a solution of H2SO4 and H2O2 (3:1, v/v) prior to use. Synthesis of LZH-DS. Layered zinc hydroxide with intercalated dodecyl sulfate (LZH-DS) was prepared by coprecipitation.22 Typically, 50 mL of NaOH (0.75 mol L 1, 37.5 mmol) was added dropwise to a vigorously stirred solution of 200 mL of Zn(NO3)2 (0.35 mol L 1, 70 mmol) containing 10.5 mmol of sodium dodecyl sulfate over 1 h at room temperature. The resulting white product was centrifuged, washed thoroughly with water, and air-dried at room temperature. The elemental and thermogravimetric analyses confirmed the chemical composition as Zn5(OH)8(DS)2 3 2H2O. Synthesis of ZnO. A 0.5 g amount of LZH-DS was dispersed into 25 mL of nBuOH (or other alcohol) under stirring at a given temperature for 24 h (the treatment time was prolonged to 72 h at 50 °C). The produced dispersion of ZnO was centrifuged; the supernatant was decanted (washed twice to remove dissolved DS); and the resulting white solid was redispersed in 10 mL of nBuOH or chloroform. The starting LZH-DS was completely transformed into ZnO as confirmed by XRD (JCPDS 01089-0510) (see the Supporting Information for more details, Figure S13). The ZnO films were prepared using the dispersions produced at 60 °C in nBuOH followed by redispersion in chloroform (up to 20 mg mL 1). The films were dip-coated (0.3 mm s 1) onto quartz plates. Instrumental Methods. Powder X-ray diffraction patterns (XRD) of the ZnO films were collected with a PANalytical X’Pert PRO diffractometer equipped with a conventional X-ray tube (Co Kα radiation, 40 kV, 30 mA, line focus) and a multichannel detector X’Celerator with an antiscatter shield. The X-ray patterns were measured in the range between 30 and 90° (2 Θ) with a step of 0.0334° and 1000 s counting time per step. We used the conventional Bragg Brentano geometry with a 0.04 rad Soller slit, 1° divergence slit, 2° antiscatter slit, and 15 mm mask in the incident beam and a 0.04 rad Soller slit, 6.6 mm antiscatter slit, and Fe beta-filter in the diffracted beam. The XRD patterns of the ZnO powders were collected using a PANalytical X’Pert PRO diffractometer equipped with a conventional X-ray tube (Cu Kα 40 kV, 30 mA, line focus) in transmission mode (see the Supporting Information for more details). The qualitative analysis was performed with the HighScorePlus software package (PANalytical, Almelo, The Netherlands, version 3.0d) and the JCPDS PDF-2 database.24 The computation of the coherent domain size based on the Scherrer formula was performed using BGMN software.25 The line profiles were corrected for instrumental broadening. Small-angle X-ray scattering (SAXS) experiments were performed using a pinhole camera (Molecular Metrology SAXS System) attached to a microfocused X-ray beam generator (Osmic MicroMax 002) operating at 45 kV and 0.66 mA (30 W). The camera was equipped with a multiwire, gas-filled area detector with an active area diameter of 20 cm (Gabriel design). Two experimental setups were used to cover the q range of 0.005 1.1 Å 1 (q = (4π/λ)sin Θ, where λ = 1.54 Å is the wavelength and 2Θ is the scattering angle). The scattering intensities were cast into the absolute scale using a glassy carbon standard. Surface imaging was performed using an atomic force microscope (AFM) (Nanoscope IIIa Multimode; Veeco/Bruker, USA) in the tapping mode with standard Si cantilevers OTESPA (Bruker, USA) with a resonant frequency ∼300 kHz.
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Table 1. Average Size of the Crystallographic Domains in the [100] (Lateral) and [001] (Thickness) Directions for the ZnO Nanoparticles Prepared by the Transformation of LZH-DSa solvent
lateral/nm
thickness/nm
b
MeOH EtOHb
10
9
iPrOHb
14
10
nBuOHb
17
9
nHxOHc nOctOHc
25 25
21 13
a
Line profile analysis of the XRD patterns was performed using the Scherrer formula as implemented in the BGMN software. b Stirring at 60 °C for 24 h. c Stirring at 80 °C for 24 h.
Table 2. Average Size of the ZnO Nanoparticles Prepared by Stirring LZH-DS in nBuOH for 24 h as Determined by XRD and SAXS powder XRDa
a
temperature/°C
lateral/nm
60 80
SAXS
thickness/nm
lateral/nm
thickness/nm
17
9
17
2.3
18
14
23
2.5
100
22
19
18
1.5
60b
12
9
16
2.2
See Table 1 for more details. b Shaking at 60 °C for 24 h.
The imaging was also performed in the contact mode with triangular silicon-nitride cantilevers NPS (Bruker, USA) with a tip sharpened by ion milling. To avoid aggregation, the samples were prepared by drop deposition of diluted colloids onto a mica support. High-resolution transmission electron microscopy (HRTEM) measurements were carried out on a JEOL JEM 3010 microscope operating at 300 kV (LaB6 cathode, point resolution = 1.7 Å). Thermal analyses were carried out on a Setaram SETSYS Evolution-16-MS instrument coupled with a mass-spectroscopy system. The measurements were performed in a synthetic air atmosphere (flow rate 30 mL/min) from 30 to 1300 °C with a heating rate of 5 °C/min. The FTIR spectra were collected on a Nicolet NEXUS 670-FT spectrometer in KBr pellets. The UV visible absorption spectra of the films on quartz plates were recorded on a Perkin-Elmer Lambda 35 spectrometer. The luminescence properties of the films were monitored on a Fluorolog 3 spectrometer (Horiba Jobin Yvon) with a cooled TBX-05-C photon detection module.
’ RESULTS AND DISCUSSION The preparation of ZnO from LZH-DS was optimized using several solvents (Table 1). The thermal treatment in MeOH did not yield ZnO. In nHxOH and nOctOH, the complete transformation of LZH-DS to ZnO was achieved at 80 °C. This transformation leads to an increase in the size of the ZnO domains as indicated by XRD; however, only the ZnO nanoparticles obtained in nBuOH and nOctOH exhibited size anisotropy. Because nBuOH is the most benign solvent that allows the formation of anisotropic ZnO, this solvent was chosen for all subsequent experiments. 24703
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Figure 1. HRTEM observations of the ZnO nanosheets prepared at 100 °C in nBuOH: (a) bright-field image with the electron diffraction pattern in the inset and (b) high-resolution image of a single crystal viewed along the [001] direction with FFT in the inset.
Figure 3. Nanomorphological composition of ZnO prepared at 60 °C shows aggregation in 3D (red and green markers) with a height of approximately 1.5 1.6 nm for a two-sheet aggregate, while a single nanosheet (black marker) is indicated by an axial height of 0.7 nm. The aggregate shows lateral growth with a lateral measure of >20 nm/unit.
Figure 2. HRTEM observations of the ZnO nanosheets prepared at 60 °C in nBuOH: (a) bright-field image with the electron diffraction pattern in the inset and (b) high-resolution image of a single crystal viewed along the [001] direction with FFT in the inset.
The influence of temperature on the nanoparticle morphology was investigated by HRTEM, AFM, and XRD (Table 2). The lowest temperature required for the transformation of LZH-DS to ZnO was 50 °C; however, a longer reaction time (72 h) was needed for the complete transformation. At temperatures of 60 °C and above, the transformation to ZnO occurred within 24 h. The HRTEM observations in Figure 1 and Figure 2 show that the as-prepared ZnO is pure, dominated by hexagonal particles with a narrow size distribution (see also Figure S1 in the Supporting Information). The FFT analysis indicates a singlecrystal structure with (001) top/bottom surfaces. The lateral size of the ZnO nanoparticles prepared at 60, 80, and 100 °C ranged between 15 and 25 nm without a clear trend in the size as documented by the XRD data in Table 2. Shaking of the LZH-DS dispersion instead of stirring led to crystals of a similar size and orientation, but the crystal surfaces were significantly rougher when shaken, probably due to the fact that ZnO nanoparticles grew from the disintegrated ones (Figure S2 in the Supporting Information). The ZnO nanoparticles were also scanned by AFM. Interestingly, the size of the 2D and 3D objects is always “quantized” as a sum of single nanosheet units having a thickness of approximately 0.6 0.7 nm (Figure 3). It was found that ∼10% of the total amount of nanoparticles (as determined by investigation of representative surface locations) represents single units, while the rest of the particles are grown in both lateral and axial directions, i.e., combined 2D 3D aggregates. Because the thickness is given by the crystal size in the [001] direction, the single
nanosheets are composed of two (0.52 nm) or three (0.78 nm) ZnO tetrahedral layers. Thus, the observed nanoparticles can be classified into three types: (i) single nanosheets with a height of approximately 0.7 nm, (ii) planar quasi-2D aggregates formed by side-by-side nanosheet stacking, and (iii) 3D aggregates preferring either axial stacking of the nanosheets or lateral growth of two-sheet stacked aggregates (see the Supporting Information for more details). On the basis of the profile analysis of the AFM topography, the axial aggregates are composed of two, three, or several stacked nanosheet units. The purification of as-prepared ZnO by centrifugation of the dispersion, separation of the solid, and redispersion in pure nBuOH yielded colloidal solutions of ZnO at concentrations up to 20 mg mL 1 that were stable for weeks. The size and shape of the nanosized particles in the dispersions were addressed by SAXS experiments. The high scattering contrast of ZnO in nBuOH allows the scattering data of a good quality to be obtained at relatively low particle concentrations (Figure S9 in the Supporting Information). The curves measured at higher concentrations coincide, after normalization to unit concentration, with those obtained for diluted dispersions. This observation indicates that nanoparticles are stable against dilution and that interparticle interference effects are negligible for the concentrations used. The SAXS curves exhibit features typical of composite particles (see Supporting Information for the detailed analysis); the intensity decay in the low-q range provides the radius of gyration of an entire nanoparticle, whereas scattering behavior at the high-q region (shoulder at q = 0.07 0.1 Å 1) is governed by subparticle characteristics. Thus, the analysis indicates that the dispersions contain large, swollen spherical nanoparticles of ZnO whose internal structure is composed of disklike nanoparticles (Table 2). A plausible explanation for this result is the agglomeration of approximately 30 100 ZnO disklike nanoparticles into spherical clusters. The thickness as determined by XRD is significantly higher (9 19 nm) than that 24704
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The Journal of Physical Chemistry C
Figure 4. Powder XRD pattern of the ZnO films on quartz plates prepared from CHCl3 dispersions: (a) as-prepared film; (b) film treated at 300 °C for 2 h; (c) film treated at 700 °C for 2 h. The curves are shifted vertically for better clarity.
obtained by the SAXS analysis (∼2 nm) probably as a result of the seamless axial stacking of single nanosheets during the ZnO drying process as shown by AFM measurements. Commonly, the size of nanoparticles increases when prepared at higher temperatures due to more rapid crystal growth. In our case, the ZnO nanoparticles are formed by the transformation of zinc hydroxide nanosheets after the delamination of LZH-DS; therefore, the size of the ZnO is primarily governed by the size of the hydroxide nanosheets.22 This mechanism allows for the preservation of {001} facets. If crystallization from a solution or dissolution redeposition were part of the processes, the resulting shape of the nanoparticles would not be nanosheets but rather needles prolonged in the [001] direction.22 As confirmed by FTIR, the surface of the ZnO nanosheets is partially covered by adsorbed DS molecules that were originally intercalated in the parent LZH-DS. The three peaks at 2957, 2922, and 2852 cm 1 correspond to the long alkyl chain, and the peaks at 1234 and 1063 cm 1 are due to the asymmetric and symmetric vibrations of the sulfate groups (Figure S10 in the Supporting Information). The presence of adsorbed DS was also verified by X-ray photoelectron spectroscopy giving the atomic ratio Zn/S ∼ 13 (i.e., Zn/DS ∼ 13) (the data not given). These results indicate a low occupancy of the ZnO surface by dodecyl sulfate molecules. To complement the results, the thermal analysis of the ZnO nanoparticles revealed a total mass loss of 13% that is attributable to adsorbed water (∼8%) and to residual adsorbed DS (
