Controlling Morphologies and Tuning the Related Properties of Nano

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Controlling Morphologies and Tuning the Related Properties of Nano/Microstructured ZnO Crystallites Xi-Guang Han, Hui-Zhong He, Qin Kuang,* Xi Zhou, Xian-Hua Zhang, Tao Xu, Zhao-Xiong Xie,* and Lan-Sun Zheng State Key Laboratory for Physical Chemistry of Solid Surfaces & Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China ReceiVed: September 16, 2008; ReVised Manuscript ReceiVed: NoVember 2, 2008

In this paper, we successfully synthesized three kinds of typical ZnO micro/nanocrystallites including flakes, columns, and pyramids by means of different facile wet chemical routes. The growth environment plays a crucial role in the morphologies of these ZnO micro/nanocrystallites. At the same time, we find that physical/ chemical properties of these ZnO samples are dependent on their exposed surface, and the order of gas sensing and photocatalytic efficiency of the ZnO crystal planes is (0001) > {101j0} > {101j1} and (0001j). On the basis of structural analyses of various exposed surfaces and related X-ray photoelectron spectroscopy, we deeply discussed the effect of definite surface structures on their gas sensing and photocatalytic properties. 1. Introduction Anisotropy is a basic property of single crystals. Various facets or surfaces have different geometric electronic structures and dangling bonds, etc. Therefore, exploring the relationship between crystal planes of solid materials and the physical/ chemical properties has attracted increasing research interests.1-15 Surface scientists, in the past several decades, have devoted intensive effort in surface-related research concerning bulk single crystals. However, the studies of surface dependent properties of small crystallites, which are of practical uses, are relatively rare. Recently, some studies concerning the relationship between the crystal surfaces of metal and their physical/chemical properties have been carried out due to successful controlled syntheses of various single-crystalline surfaces and their important applications in catalytic reactions.1-10 On the contrary, the systematical study about the connection between the crystal planes of metal oxide and property is not often reported, probably due to poor morphology-controlled syntheses of nanoscaled metal oxides.11-15 Therefore it is still a great challenge to deeply investigate the dependence of physical/ chemical properties of metal oxide nanomaterials on the definite crystal surface. Among various physical/chemical properties, gas sensing and photocatalytic reaction occurs at the crystal surface.16-18 As a result, the efficiency of gas sensing and photocatalysis should strongly depend on the surface structure. Zinc oxide (ZnO), a typical n-type wide band gap semiconductor (Eg ) 3.26 eV), plays an important role in many application fields from optoelectronics to energy conversion, gas sensing, and photocatalysis.17-28 In this paper, we report the shape-controlled syntheses of ZnO and corresponding gas sensing and photocatalytic properties. Three kinds of typical ZnO micro/nanocrystallites including flakes, columns, and pyramids are successfully synthesized by different facile wet chemical routes. Electron microscopies have been applied to characterize the surface structures of the as-prepared three kinds of ZnO micro/nanocrystallites, and the gas sensing properties and photocatalytic activities have been measured. On the basis * Corresponding authors. E-mail: [email protected], [email protected]. Fax: +86-592-2183047. Tel.: +86-592-2180627.

of structural analysis of various crystal planes and related X-ray photoelectron spectroscopy, we deeply discussed the effect of surface structure on the gas sensing and photocatalytic properties. 2. Experimental Section 2.1. Chemicals and Reagents. Zinc acetate (Zn(CO2CH3)2 · 2H2O, 99%), ethylenediamine (C2H8N2, 99%), 1-octylamine (C8H19N, 99%), oleic acid (OA, 90%), sodium hydroxide (NaOH, 96%), glycerin (C3H8O3, 99%), zinc sulfate (ZnSO4 · 4H2O, 99.5%), sodium fluoride (NaF, 98%), and methyl orange (MO, analytical reagent) were used. All chemicals used in the experiments were purchased from commercial suppliers (Alfa Aesar and Sinopharm Chemical Regent Co., Ltd.) and used as received without further purification. 2.2. Synthetic Methods. 2.2.1. Synthesis of ZnO Flakes. ZnO flakes were prepared either by the technique reported by Jang et al.17 or via a hydrothermal route described as below. In a typical experiment, 0.287 g of ZnSO4 · 4H2O, 0.080 g of NaOH, and 0.083 g of NaF were successively added into a mixed solvent of 7 mL of distilled water and 3 mL of ethanol. The resulting mixture was sonicated for several minutes and transferred into a Teflon-lined stainless-steel autoclave with a capacity of about 20 mL. Then the autoclave was heated to 200 °C and held for 24 h. After the mixture was cooled to room temperature, the production was separated by centrifugation from the solution and repeatedly rinsed with distilled water. 2.2.2. Synthesis of ZnO Columns. A total of 0.66 g Zn(CO2CH3)2 · 2H2O and 2.4 g NaOH was dissolved in 9 mL of distilled water. Then 21 mL of glycerin was dropped to the above solution. After magnetic stirring for 15 min, the mixture was transferred into a Teflon-lined stainless-steel autoclave and was kept at 150 °C for 24 h. The products were collected by centrifugation at 4000 rpm and washed several times with deionized water and ethanol. 2.2.3. Synthesis of ZnO Pyramids. ZnO pyramids were synthesized by thermal decomposition of Zn(CO2CH3)2 in the mixed solvent of organic amine and carboxylic acid, which was reported in detail in our previous work.29 In a typical experiment, 1.46 g anhydrous zinc acetate (Zn(CO2CH3)2) was added into a mixed solvent of 6.6 mL of OA and 4.5 mL of ethylenediamine.

10.1021/jp808233e CCC: $40.75  2009 American Chemical Society Published on Web 12/16/2008

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Figure 1. Typical SEM images of three kinds of ZnO samples with various morphologies: (A) ZnO flakes, (B) ZnO columns, and (C) ZnO pyramids.

Figure 2. XRD patterns of three kinds of ZnO samples with various morphologies.

The resulting mixture was transferred into a glass tube with the upper end open, then rapidly heated to 286 °C within 10∼15 min, and held for 1 h. After reaction, the precipitates were collected and washed several times with hexane and ethanol. 2.3. Characterization and Measurements. 2.3.1. Characterization of the Morphology and Structure. The composition and phase of as-prepared products were acquired by the powder X-ray diffraction (XRD) pattern using a Panalytical X-pert diffractometer with Cu KR radiation. The morphology and crystal structure of as-prepared products were observed by scanning electron microscopy (SEM, Leo 1530) and highresolution transmission electron microscopy (HRTEM, FEI Tecnai-F30) with an acceleration voltage of 300 kV. All TEM samples were prepared from depositing a drop of diluted suspensions in ethanol on a carbon film coated copper grid. The surface compositions of ZnO samples were determined by a PHI QUANTUM2000 photoelectron spectrometer (XPS) using a monochromatic magnesium X-ray source. The binding energies were calibrated with respect to the signal for adventitious carbon (binding energy of 284.6 eV). 2.3.2. Measurement of Gas Sensing Properties. The gas sensors were fabricated by dropping a proper amount of the paste consisting of the ZnO nano/microstructures and absolute ethanol on alumina ceramic tubes mounted with two Au electrodes. After the evaporation of ethanol, the alumina ceramic tubes were coated by a layer of homogeneous ZnO film. A small Pt-Au alloy coil was placed through the tubes as a heater, which was able to provide the working temperature from 200∼500 °C for the gas sensors. Before the measurement, the sensors were aged at 400 °C for one day to achieve stabilization. The gas sensing tests were operated in a measurement system of WS-30A (Zhengzhou Winsen Electronics Technology Co. Ltd., PR China). C2H5OH as the detecting target was injected into a test chamber and mixed with air after complete thermal

evaporation. The resistances of the sensors were measured under a bias voltage of 5 V DC. The sensitivity of the sensor was defined as the ratio of the resistance in air (Rair) to that in ethanol gas (Rgas), i.e., Rair/Rgas. All the tests were operated under about 50% RH. 2.3.3. Measurement of Photocatalytic Properties. The photodegradation efficiency of MO in aqueous solution was measured under the UV irradiation (300 W Hg lamp, 365 nm). All the experiments were carried out at the temperature of 25 ( 2 °C. Typically, 0.0050 g of ZnO samples were put into a series of 10 mL Pyrex glass vessels containing of 5 mL of MO (10 mg/L) and were magnetically stirred in the dark to achieve the adsorption equilibrium of MO on the catalysts before exposure to UV irradiation. Analytical samples were drawn from the Pyrex glass vessels at intervals of 20 min and centrifuged to remove the ZnO photocatalysts. The photodegradation efficiency was monitored by measuring the absorbance of the centrifuged solutions at its maximum absorption wavelength of 465.5 nm with UV-vis spectroscopy (SHIMADZU, UV-2100) at room temperature. The concentration of MO as a function of time was calculated by the absorbency values of the original and measured samples. 3. Results and Discussion 3.1. Morphology and Structure of As-Synthesized ZnO Samples. Structurally, wurtzite-structured ZnO crystal can be described as a number of alternating planes composed of tetrahedrally coordinated O2- and Zn2+ ions, stacked alternatively along the c-axis.28 Such structural characters result in the diverge of surface energy of polar (0001) surface, and a strong anisotropy in the growth rate ν, such as ν[0001] . ν[101j0].30 Hence wurtzite-type ZnO nanostructures usually tend to maximize the exposed areas of the {101j0} nonpolar facets and minimize the exposed areas of the {0001} polar facets with high surface energy. To tune the morphology and exposed surfaces, the growth of ZnO nanocrystals can be realized by controlling the growth environments.29,31,32 Figure 1 shows typical SEM images of three kinds of ZnO samples grown in different environments. Sample 1 (Figure 1A), which is prepared by a hydrothermal route from ZnSO4 · 4H2O in the presence of large amount of NaF, is of flake-shaped structures. Such flake-shaped structures are about 300∼400 nm in width and about 20-30 nm in thickness. Sample 2 (Figure 1B) prepared by another hydrothermal route from Zn(CO2CH3)2 · 2H2O is high-yield columns having a defined hexagonal cross section. The length of these columns is about 30∼50 µm, and the diameter is about 400 nm to 1 µm. Sample 3 (Figure 1C) prepared by thermal decomposition of Zn(CO2CH3)2 in the mixed solvent of OA and ethylenediamine

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Figure 3. (A) TEM image of ZnO flakes and (B) TEM image of an individual ZnO flake, (C) corresponding HRTEM image recorded from the area marked in (B); (D) TEM image and (E) SAED pattern of an individual ZnO column, (F) corresponding HRTEM image recorded from the area marked in (D); (G) TEM image and (H) SAED pattern of an individual ZnO pyramids, (I) corresponding HRTEM image recorded from the area marked in (G).

represents the morphology of hexagonal pyramids, and their base size is in the range of 300 nm to 1.5 µm. The XRD patterns of these samples are shown in Figure 2. It is found that all as-prepared samples are highly crystalline, and the diffraction peaks in every pattern can be indexed as hexagonal wurtzite-type ZnO (JCPDS No. 36-1451). No peaks due to impurities are detected, indicating that all zinc salt precursors have been thoroughly decomposed into pure ZnO during the reaction. Further HRTEM and SAED characterizations provide us more useful information about the structural details of these ZnO samples. As shown in Figure 3A,B, most of the ZnO flakes have a regular hexagonal outline. The corresponding HRTEM image in Figure 3C indicates that the ZnO flake exhibits clear lattice fringes with the lattice spacing of 0.281 nm, corresponding to the distance between two {101j0} crystal planes of wurtzite ZnO. The fast Fourier transform (FFT) pattern (the inset of Figure 3C) accords with the diffraction pattern of the [0001] zone axis of ZnO. Therefore, the bottom/top surfaces of the ZnO nanoflakes are ((0001) planes. Typical TEM image of an individual ZnO column is shown in Figure 3D, the diameter of which is about 300 nm. The SAED pattern (Figure 3E) and corresponding HRTEM image (Figure 3F) reveal that the ZnO columns grow along the [0001] direction and their side surfaces are nonpolar {101j0} planes. Typical TEM image of an individual ZnO pyramid with the diameter of 500 nm is shown in Figure 3G. This pyramid locates on the copper grid with the top upward, resulting in a regular hexagonal projecting outline. From the SAED pattern (Figure 3H) and corresponding HRTEM

image (Figure 3I), it can be found that ZnO pyramids are single crystals and the pyramidal tip direction is along the [0001] direction. According to our previous studies,29 the base surface of ZnO micropyramids is the (0001j) plane and the side surfaces of mico-pyramid are the {101j1} planes. Above structural characterization results demonstrate that three kinds of ZnO samples prepared via different synthetic routes have different exposed surfaces. The ZnO columns have their nonpolar {101j0} planes exposed. For ZnO nanoflakes, the dominating surfaces are ((0001) planes. For ZnO columns and pyramids, however, the dominating surfaces are {101j0} and {101j1}, respectively. The above results demonstrate that the growth environment has a significant influence on the final morphology of ZnO crystals. As we know, wurtzite ZnO has two groups of polar planes, the {0001} and {101j1} planes. Under normal conditions, nonpolar planes (such as {101j0} and {21j1j0} planes) tend to be exposed because of their low surface energies. The hexagonal column shown in Figure 1B is a representative morphology of ZnO crystallites with nonpolar surfaces. In ionic environments (such as high-concentration NaF solution and organic amine/ carboxylic acid mixed solution used in our experiments), however, the growth of those polar planes would be suppressed because their surface energy can be greatly reduced by electrostatic interactions with positive/negative ions in the solution.29 As a result, some special morphologies (such as flakes and pyramids) exposed with polar planes could be obtained in ionic environments. In our present study, successful morphology-controlled syntheses of ZnO micro/ nanocrystallites provide

Morphologies of ZnO Micro/Nanocrystallites

Figure 4. (A) Curves of gas sensitivity versus working temperature by using 300 ppm of ethanol as the detecting gas. (B) The photodegradation plots of the MO (the initial concentration of MO is 10.0 mg/ L) without or with adding ZnO catalysts. (C) The photocatalytic efficiencies of different ZnO samples after normalization with the surface area of the ZnO samples.

an opportunity to systematically investigate the relation between exposed planes of ZnO nanocrystals and related physicochemical properties. 3.2. Gas Sensing and Photocatalytic Properties of AsSynthesized ZnO Samples. Figure 4A presents gas-sensing properties of three kinds of ZnO samples, showing the sensitivity changes of three kinds of ZnO samples as a function of the working temperature in the environments of 300 ppm alcohol. It can be seen that all ZnO samples have the highest response sensitivity at a working temperature of 350 °C. At the same time, the sensitivity of ZnO flakes is found to be much higher than those of two other kinds of ZnO samples. The sensitivity of ZnO flakes reaches 75.7 at the optimal working temperature of 350 °C while the sensitivity of ZnO columns is 31.5, and the sensitivity of ZnO pyramids is only about 9.3 at the same working temperature. Figure 4B shows photodegradation plots of the MO with and without the presence of three kinds of asprepared ZnO samples. In comparison with slow degradation process of the MO under the UV irradiation, the adding of ZnO catalysts markedly accelerated the degradation rate of the MO. In order to more clearly show the influence of different exposed surfaces on the photocatalytic ability of ZnO, the photocatalytic

J. Phys. Chem. C, Vol. 113, No. 2, 2009 587 efficiencies of three kinds of ZnO samples are normalized by the BET surface areas, as shown in Figure 4C. The axis of y in Figure 4C is defined as the transform quantity of the MO per surface area of ZnO catalysts, i.e., (C0 - C)/S (C0 and C: initial concentration and ultimate concentration of MO, respectively; S: specific surface area of the sample). From Figure 4C, it can be seen that the surface degradation efficiency of MO under the UV illumination is in the order of flakes > columns > pyramids, and thus the surface of the flakes has the highest photocatalytic activity. Interestingly, the order of surface degradation efficiency for three ZnO samples is consistent with the order obtained from the gas sensing tests shown in Figure 4A. Considering the different dominating surfaces of the three kinds of ZnO samples, it can be concluded that the gas sensing and photocatalytic abilities of ZnO micro/nanocrystallites are closely related to those of exposed surface structures. 3.3. Relation between Exposed Surfaces of ZnO Samples and Their Gas Sensing and Photocatalytic Properties. In principle, gas sensing of a metal oxide semiconductor is a solid-gas interfacial reaction process, which produces an intense change of the resistance of metal oxide semiconductor. Photodegradation of organic pollutants is also a surface oxidationreduction process driven by photogenerated electron-hole pairs. Therefore the chemical adsorption and reaction of target molecules occurring on the surface of metal oxide semiconductors is the most crucial factor to the gas sensing and photocatalytic behavior. In the past years, great efforts have been made to investigate the influence of the morphology, size, and surface area of metal oxide nanostructures on their gas sensing and photocatalytic properties.33-36Recent studies reveal that the surface structures and composition might be the essential factor to determine the efficiency of gas sensing and photocatalytic properties.17,18 In order to obtain useful information about surface structures of as-prepared ZnO samples, XPS analyses were used. Figure 5A shows Zn 2p XPS peaks of ZnO flakes, columns, and pyramids for comparison. It is found that the Zn 2p XPS peaks of three ZnO samples are analogous for their position and distribution. However, there are some slight differences between O 1s XPS peaks of three ZnO samples, which are asymmetrical and present a visible shoulder. As shown in Figure 5B-D, each O 1s XPS peak can be decomposed into three Gaussian components centered in about 530.1 eV (OL), 531.5 eV (OV), and 532.5 eV (OC) by fitting with the Gaussian function. According to the references,37,38 the OL component of O 1s spectrum is attributed to O2- ions in ZnO lattice, the OV component at the medium binding energy is associated with O2- ions in oxygen-deficient regions within the matrix of ZnO, and the OC component is usually attributed to chemisorbed and dissociated oxygen species or OH. Therefore, we can estimate the oxygen-chemisorbed ability of different exposed planes in ZnO crystal according to the intensity of OC component in the O 1s XPS peak. As shown in Table 1, the relative percentages of the OC component in three different samples are about 16.3% (flakes), 8.4% (columns), and 3.5% (pyramids), respectively. That is to say, the ZnO flakes may absorb more oxygen species, while the ZnO columns do worse, and ZnO pyramids do the worst. Such order of chemisorbed ability of these ZnO samples is consistent with that of their gas sensing and photocatalytic abilities, which adequately demonstrates that the gas sensing and photocatalytic properties of ZnO are closely related to the chemisorbed ability of the crystal surfaces. To explain the discrepancy in the chemisorbed ability of ZnO with different morphologies, structural analyses of the exposed

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Figure 5. (A) Zn 2p XPS spectra and (B-D) O 1s XPS spectra of three kinds of ZnO samples: (B) flakes, (C) columns, and (D) pyramids.

Figure 6. (A) Atomic stacking model of wurtzite ZnO project along [12j10], clearly showing ((0001), {101j0}, {101j1} planes. (B, C, D) Schematic illustration of the morphology models for flakes, columns, and pyramids, respectively.

TABLE 1: Results of Curve Fitting of O 1s XPS Spectra of Different ZnO Samples ZnO samples

OL OV OC Zn 2p (Zn-O) (vacancy) (chemisorbed)

flakes binding energy (eV) 1021.36 relative percentage (%) columns binding energy (eV) 1021.49 relative percentage (%) pyramids binding energy (eV) 1021.73 relative percentage (%)

530.11 60.4

531.44 23.3

532.51 16.3

530.14 77.3

531.51 14.3

532.53 8.4

530.19 62.9

531.52 33.6

532.59 3.5

surfaces are very helpful. Figure 6A is an atomic stacking model of wurtzite ZnO projected along [12j10] direction, clearly showing the atom arrangements in {0001}, {101j0}, and {101j1} planes. In the structure of wurtzite ZnO, O2- and Zn2+ ions

stack alternatively along the c-axis, resulting in that the Znterminated (0001) plane and the O-terminated (0001j) plane become polarized. Similarly, the {101j1} planes of wurtzite ZnO are another group of polar planes with higher surface energy. The {101j0} planes are composed of equivalent O2- and Zn2+ ions at the same planes, and thus they are the nonpolar planes with the lowest surface energy. Figure 6B-D show the morphology models of as-prepared ZnO flakes, columns, and pyramids, respectively. For ZnO flakes, the dominating planes are the Zn-terminated (0001) plane (i.e., top surface) and the O-terminated (0001j) plane (i.e., bottom surface). For ZnO columns, the dominating planes are nonpolar (101j0) planes (i.e., six side surfaces). For ZnO pyramids, the mainly exposed planes are the O-terminated polar (0001j) plane (bottom surface) and the {101j1} planes (six side surfaces), both of which are chemically inert.

Morphologies of ZnO Micro/Nanocrystallites The difference of surface atomic structures should result in a distinct ability to absorb the oxygen species (such as O2, O2-, O-, OH-) and target molecules. As far as the (0001) plane terminated with Zn2+ ions is concerned, its chemsorption ability is the best because that Zn2+ ions on the exposed (0001) surface are able to seize atmosphere oxygen (O2) through physical/ chemical absorption due to unsaturated oxygen coordination. In the solution environment of photodegradation, based on similar theory, the (0001) surface terminated with Zn2+ ions is facile to adsorb O2 and OH-. The adsorbed O2 and OHrespectively combine with photogenerated electrons (e-) and holes (h+) and then produce more high-activity O2•- and OH•, which can promote the photocatalysis reaction. For the (0001j) plane and {101j1} planes terminated with O2- ions, the ability to absorb oxygen is very weak because terminated O2- ions are impossible to further absorb oxygen molecules or other oxygen species. Among the three kinds of ZnO presented in this paper, half-surfaces of ZnO flakes are the Zn-terminated (0001) plane and thus their gas sensing and photocatalytic properties are markedly enhanced, while all surfaces of ZnO pyramids are the O-terminated planes and thus their related properties are depressed to a certain extent. For ZnO columns, the dominating exposed surfaces of them are the nonpolar {101j0} planes with equivalent Zn atoms and O atoms in the same plane, so their gas sensing and photocatalytic properties are medium. On the basis of the discussion above, it can be concluded in principle that the order of gas sensing and photocatalytic activity of the ZnO crystal faces is (0001) > {101j0} > {101j1} and (0001j), which agrees well with our experimental results shown in Figure 4. 4. Conclusion Three kinds of ZnO micro/nano structures with different definite crystal surfaces have been successfully synthesized, and the relationship between surface structures and gas sensing and photocatalytic properties have been studied. We find that the order of gas sensing and photocatalytic efficiency of the ZnO crystal planes is (0001) > {101j0} > {101j1} and (0001j). On the basis of related XPS spectra and structural analysis, we believe that the gas sensing and photocatalytic efficiency of these ZnO samples with different morphologies depends on the chemsorption ability of the exposed planes. The Zn terminated surfaces have the highest chemsorption ability and therefore have the highest gas sensing and photocatalytic activity. This study should be helpful for fully comprehending the anisotropy of metal oxide nanocrystals and improving their gas sensing and photocatalysis efficiency. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grants 20725310, 20721001, 20673085, and 20801045) and the National Basic Research Program of China (Grants 2007CB815303 and 2009CB939804).

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