Morphology−Function Relationship of ZnO: Polar Planes, Oxygen

J. Phys. Chem. C 2008, 112, 11859–11864

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Morphology-Function Relationship of ZnO: Polar Planes, Oxygen Vacancies, and Activity G. R. Li,† T. Hu,† G. L. Pan,† T. Y. Yan,† X. P. Gao,*,† and H. Y. Zhu*,‡ Institute of New Energy Material Chemistry, Department of Materials Chemistry, Nankai UniVersity, Tianjin 300071, China, and School of Physical and Chemical Sciences, Queensland UniVersity of Technology, Brisbane, Queensland 4001, Australia ReceiVed: February 18, 2008; ReVised Manuscript ReceiVed: May 29, 2008

The relationship between morphology and function of ZnO is demonstrated by investigating its polar planes, oxygen vacancies, and catalytic activity for N-formylation. ZnO with various morphologies is controllably synthesized via simple hydrothermal reactions. Scanning electron microcopy images exhibit a variety of the as-prepared hexagonal zinc oxides: rods, disks, rings, and screw caps as a new member of ZnO morphology family. Each of the morphologies is remarkably different from the others in the proportion of the (0001) and (0001j) polar planes in the outside surfaces of ZnO crystals. The analysis of photoluminescence spectra shows that there exist more oxygen vacancies in the samples with large polar planes. The synthesized samples are used as a catalyst for the N-formylation of aniline and show a morphology-dependent activity: ZnO with large polar planes is more catalytically active for the N-formylation reaction. This is attributed to the fact that the polar planes generate easily oxygen vacancies, which are considered as the favored sites for catalyzing the N-formylation reaction. The results suggest a positive relationship among polar planes, oxygen vacancies, and catalytic activity for N-formylation. Introduction Morphology-controlled synthesis of inorganic materials on a nano/micrometer scale has achieved much progress in the past decade. Understanding the morphology-structure-function relationship is of considerable importance for further fabricating highly functional materials for practical applications.1 Among several attractive materials studied so far, ZnO exhibits the widest varieties of nano/micromorphologies ranging from tubes,2 wires,3 rods,2,4 rings,5 belts,5,6 cones,7 and orientation arrays8 to more complicated structures such as micropyramids,9 rotors,10 boxes,11 drums, cages,12 and towers.13 Each of these exceptional morphologies has its own unique features in terms of orientation, size, or both, which could have a crucial effect on certain properties. Some cases have shown that the morphology of ZnO plays an important role in determining properties. The aspect ratio of ZnO nanobelts influences strongly the elastic modulus due to the presence of stacking faults in nanobelts growing along particular directions.14 The optical properties of ZnO depend especially on its nanostructures, as reviewed in the literature.15 However, for the attractive material, there are still a great deal of unanswered questions regarding the relationship between morphology and properties. In terms of structural properties, ZnO is a polar crystal with a number of alternating planes composed of tetrahedrally coordinated O2- and Zn2+ ions, stacked alternately along the c-axis.16 Thus, a ZnO crystal consists of a positively charged plane (0001) terminated with zinc, a negatively charged plane (0001j) terminated with oxygen, and a nonpolar plane (011j 0) with C6V symmetry.16 Among various synthesis methods, hydrothermal synthesis is dominantly used because of its relative mildness and simplicity. ZnO tends to form one-dimensional structures in the hydrothermal process, since the crystal growth * To whom correspondence should be addressed. E-mail: xpgao@ nankai.edu.cn; [email protected]. † Nankai University. ‡ Queensland University of Technology.

is faster along the direction than along other directions.4,17 The crystal growth habit can be modified by selective adsorption of additives on the planes of the polar crystals. Some additives, such as citrate ions,18 CTAB,19 and polymers,20,21 have been introduced into hydrothermal systems to fabricate exceptional shapes. In this regard, hexagonal disks and rings of ZnO were synthesized by a solution-phase process including microemulsion,22 in which sodium bis(2-ethylhexyl) sulfosuccinate was used to restrain the growth along c-axis. Subsequently, similar hexagonal disks were also obtained from other routes.23 Recently, Peng and co-workers fabricated ZnO hexagonal disks and rings using a hydrothermal reaction with polyacrylamide as an additive.24 The N-formylation reaction generates formamides that are important intermediates in organic synthesis, especially in the synthesis of pharmaceutically active compounds.25 The Nformylation of lysine is even related to oxidative DNA damage.26 Many methods available for N-formylation of amines are limited because the formylation reagents are toxic or expensive and the operation requires strictly anhydrous conditions.25 Recently, ZnO was used as a new catalyst for N-formylation reactions under solvent-free conditions with formic acid as formylation reagent.27 Obviously, this is valuable for extending the pharmaceutical and biotechnical application of N-formylation because ZnO is biosafe and biocompatible.28 Therefore, further understanding of the relationship between ZnO morphology and the catalytic activity for N-formylation is significant. In the present study, we synthesize ZnO with various morphologies via a facile hydrothermal process and investigate the effect of morphology on the catalytic activity for Nformylation of aniline. It is found that the polar planes of ZnO are important for the activity, because they favor forming more oxygen vacancies responsible for catalyzing the N-formylation reaction. In addition, the growth process of the various ZnO morphologies is also discussed.

10.1021/jp8038626 CCC: $40.75  2008 American Chemical Society Published on Web 07/15/2008

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Experimental Section Preparation. In a general procedure for synthesizing ZnO rods and disks, the same molar amounts of zinc acetate dihydrate (ZnAc2, Zn(CO2CH3)2 · 2H2O) and hexamethylenetetramine (HMT, C6H12N4) were dissolved in 24 mL of deionized water. After a stirring of 10 min, the solution was transferred into a 60-mL Teflon-lined autoclave and maintained at 97 °C for 12 h. The white precipitate was collected after the autoclave was cooled to room temperature and was washed with deionized water. ZnO rods were obtained at a relatively low concentration of zinc ions (0.01 mol/L, ZnAc2:HMT ) 1:1). ZnO hexagonal disks were obtained when the concentration of ZnAc2 and HMT was increased to 0.57 mol/L (3.00 g of ZnAc2 and 1.92 g of HMT). ZnO hexagonal rings were synthesized using a procedure similar to the preparation of the disks except for that the reaction temperature was raised to 100 °C and the reaction time was prolonged to 34 h. In a secondary hydrothermal growth process for producing ZnO microscrew caps, the as-prepared ZnO rings were dispersed into the same starting solution (ZnAc2:HMT ) 1:1, 0.57 mol/L), and the mixture was kept at 100 °C for 12 h. All the samples were dried at 60 °C and then calcined at 200 °C for 2 h prior to structure characterization and catalytic test. Characterization. The morphology of the samples was characterized by scanning electron microscope (SEM, Hitachi S-3500N). The crystal structure of ZnO was confirmed by X-ray diffraction (XRD, Rigaku D/max-2500). The surface area was measured on a NOVA 2000e surface area analyzer (Quantachrome) by nitrogen absorption at -196 °C using the BrunauerEmmett-Teller (BET) method. Photoluminescence (PL) spectroscopy was performed at room temperature on an Accent RPM2000 PL mapping system using 325-nm emission from a He-Cd laser and detected through a 345-nm high-pass filter. Catalytic Activity. The N-formylation of aniline was performed in a 50-mL flask with a reflux condenser. The mixture, consisting of aniline (C6H7N, 9.300 g, 0.1 mol), formic acid (HCO2H, 13.100 g, 88%, 0.25 mol), and ZnO (2.000 g, 0.025 mol), was stirred with a magnetic stirrer at 70 °C for 30 min. Then the liquid was separated by centrifugation and analyzed by gas chromatography/mass spectrometry (HP G1800A) with an external standard method. The experimental error of yield was less than 1%. The N-formylaniline yield (Y) and selectivity (S) were defined as the following ratio, respectively:

Y)

AN AN ) × 100% AAn A’ An + AN + 2ADPFA S)

AN × 100% (1) AN + ADPFA

where AN is the amount of the N-formylaniline in the products, ADPFA is the amount of N,N′-diphenylformamidine (DPFA), the sole byproduct detected, AAn and A′An are the amount of the aniline in the reactant mixture and in the products, respectively. Results and Discussion Morphologies and Structures. Figure 1 show the representative SEM images of the ZnO samples prepared under different conditions. At a low concentration (0.01 mol/L), the hydrothermal reaction yields one-dimensional ZnO rods with a diameter of 200-500 nm and a length of 6-10 µm (Figure 1a, rods). ZnO hexagonal disks are formed when the concentration of ZnAc2 is 0.57 mol/L. According to SEM observation, the disk sample has a high purity of more than 95% without aggregation. The disks show an outer profile close to an equilateral hexagon

Figure 1. SEM images of the synthesized ZnO with various morphologies: rods (a), disks (b), rings (c, d showing a large magnification of a ring), and screw caps (e, f showing a large magnification of a screw cap).

and are 2-3 µm in diameter and ∼1 µm thick, similar to those reported in diameter but thicker (Figure 1b, disks).22,24 The ring sample contains 70-80% of ZnO hexagonal rings and some microspheres that are the aggregates of ZnO particles with irregular shapes (Figure 1c, rings). However, when we placed an indium tin oxide glass plate standing in the autoclave in the hydrothermal process, we collected ZnO rings in the upper part of the plate for characterization and catalytic test, while microspheres were in the bottom. Figure 1d shows a higher magnification view of a ring, revealing that the hexagonal ring has outer diameters of 2-3 µm similar to those of the disks, inner diameters of 1.5-2 µm, and thickness of ∼500 nm. Furthermore, the central hole of the rings is also hexagonal, corresponding to the hexagonal outer profile with the same symmetrical characteristics, so that the thickness of the ring frame is relatively uniform, being ∼500 nm. After the secondary hydrothermal growth with the as-prepared rings as seeds, the product comprises mainly screw cap-like morphologies and a small fraction of ZnO hexagonal disks (5-10%) that are very similar to those appearing in the disk sample (Figure 1e and f, screw caps). To our best knowledge, such morphology of ZnO has not been reported previously. From Figure 1f, it appears that the screw caps seem like a hexagonal ring being contained in a big outer ring. Obviously, the inner rings are the precursor as seeds with the diameter of 2-3 µm. The outer rings are 6-7 µm in diameter and ∼1 µm in wall thickness. All the diffraction peaks in XRD patterns of the samples can be indexed as ZnO crystals with a wurtzite structure (Figure 2). However, the diffraction intensity ratio of (002) polar plane to (100) nonpolar plane (I(002)/I(100)), that is given in the parentheses in Figure 2, is obviously different, having the following order: disks > screw caps > rings > rods. A high I(002)/I(100) value means a large fraction of polar planes to a large extent. Growth Process. As shown in Figure 1a and b, the ion concentration is largely responsible for the evolution of ZnO

Morphology-Function Relationship of ZnO

Figure 2. XRD patterns of the synthesized ZnO with various morphologies. The values in the parentheses are the diffraction intensity ratio of (002) polar plane and (100) plane, I(002)/I(100).

SCHEME 1: Growth Process Scheme of ZnO Crystal at Different Ion Concentrations

morphology from rods to disks. This is the competition result between nucleation and crystal growth,29 similar to the discussion in much of the literature. 22–24,30,31 On the other hand, although there are no additives in our hydrothermal reaction, acetate anions from the raw material could be adsorbed on the (0001) surface of ZnO substituting for hydroxyl anions at a relatively high concentration (Scheme 1),24 reducing largely the growth velocity in the direction and forming the hexagonal disks as shown in Figure 1b. Partial dissolution of hexagonal disks yields ZnO rings, and a relatively higher temperature and a longer reaction time, facilitate such a process. A formation mechanism of ZnO rings has been proposed by Wang and co-workers.22 The exposed negatively charged O2- (0001j) surface of the ZnO disks is considered to be reactive toward NH4+ and NH3 · H2O. Furthermore, the center of the disks has the highest defect density so that this position can be easily etched, resulting in the ring shape. The etching reaction proceeds much faster at higher temperatures.22 Our observation supports the proposed mechanism. When the ZnO disks are hydrothermally treated in a 0.2 mol/L ammonium acetate solution (100 °C, 12 h), SEM images of the specimens taken during the process show the etched profiles of the disks to different extents (Figure 3). Clearly, ZnO is easily etched from the center of the disks, and the etching starts from one of the two basal surfaces of the disks due to polarity (Figure 3f shows the smooth back surface of an etched surface). The result also reveals that the center of the disks has the highest defect density responsible for the fastest etching. The formation of the highest defect density in the center is ascribed to the

J. Phys. Chem. C, Vol. 112, No. 31, 2008 11861 growth mode of the disks. According to the screw dislocation model of crystal growth,32 lattice defects have the highest binding energy and are the most favorable positions for the incorporation of a unit molecule from the solution phase; additionally, the angular velocity near the corner of the lattice defects is faster than that at the edge of the crystal.32 Thus, the screw growth leads to the highest defect density in the center of the disks. The layer-by-layer morphology of the center hole in Figure 3b-d seems to imply the screw dislocation growth mode for ZnO disks in this study. An epitaxial growth on the synthesized rings yields the ZnO screw caps. The growth starts from the defect sites of the seed rings,32 which occurs on every surface of the hexagonal rings. In the directions, the epitaxial growth on six outer side surfaces of the rings is isotropic. Initially, the ZnO crystalline grains grow mainly along the direction due to epitaxy. However, as a result of the polar characteristic of ZnO, the growth along the direction has a gradually increasing velocity.16 Therefore, the new outer profile of the product still keeps a hexagonal shape and has the same symmetrical characteristics as the seed ring. Figure 4a shows clearly the appearance of both the new screw cap and the ring as seeds. Meanwhile, the epitaxial growth yields a new smooth (0001) surface as shown in Figure 4b. On the other hand, for the inner surfaces and the (0001j) surface of the rings, an etching reaction must be taken into account, because the hydrothermal conditions are suitable for etching the sites with high-density defects as mentioned above. Evidentially, Figure 4c shows the image of a screw cap etched seriously. As a result, we believe that the equilibrium between crystal epitaxial growth and orientated etching leads to the formation of the ZnO screw caps. Photoluminescence. The room-temperature PL spectra of ZnO with different morphologies are shown in Figure 5a. In all cases, the spectra show two bands: a luminescence band centered at 380 nm (the weak intensity is due to the effect of a 345-nm filter before the detector) and a broadband in the region of 440-840 nm that has a dominantly strong intensity. For the broad luminescence band, it is very obvious that the different samples show the following intensity order: screw caps > disks > rings > rods. For the peak centered at 380 nm, rods have a relatively weak intensity, and the other three samples have a close value. The current PL spectra are generally similar to the ZnO PL spectra detected in many works.33–36 The peak centered at 380 nm (3.26 eV) indicates the near-band-edge (3.37 eV) emission and free-exciton peak of ZnO. The broad band in the visible-light region is widely considered to result from ZnO surface detects, in which oxygen vacancies are the most suggested defects.33–36 Wu et al. investigated stoichiometric and oxygen-deficient ZnO film and found that the intensity of the green emission had a good correlation with the oxygen deficiency in the film, while the yellow emission could be assigned to the interstitial oxygen.35 Similarly, the current broadband can also be Gaussian divided into two bands in the green and the yellow range (Figure 5b-e). The intensity of the green emission attributed to oxygen vacancy varies with different ZnO morphologies, following the same order as the total broadband, screw caps > disks > rings > rods, and so does the yellow emission. This demonstrates that there are various amounts of oxygen vacancies in the ZnO samples with different morphologies, and oxygen vacancies decrease in turn from screw caps, disks, and rings to rods. The dependence of the green band emission on ZnO morphology was also observed by Andelman et al., and it was found that the strongest green

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Figure 3. SEM images of ZnO disks etched in ammonium acetate solution to different extents (a-e), and a back surface of the etched surfaces (f).

Figure 4. SEM images of ZnO screw caps showing different points of view (a, b) and etching (c).

TABLE 1: Yield and Selectivity of N-Formylaniline over the Various ZnO Catalysts, and BET Surface Area ZnO sample

BET specific surface area (m2 · g-1)

yield of N-formylaniline (%)

selectivity of N-formylaniline (%)

blank irregular particles screw caps disks rings rods

14.1 11.9 4.6 12.5 4.5

46.5 90.0 97.3 93.9 90.4 88.4

100 99.7 100 99.6 99.8 99.4

band intensity corresponded to the shape with the largest surface/ volume ratio, which involved the most amount of surface oxygen vacancies.36 Though PL spectra show an integral response of the samples, we can analyze the contribution of the particle morphology for oxygen vacancy owing to the uniformity in morphology. Based on geometrical knowledge, it can easily be concluded that the proportion of polar planes in ZnO surface decreases in turn from screw caps, disks, and rings, to rods. Interestingly, the order is consistent with the quantitive order of oxygen vacancy, suggesting there is a potential correlation between oxygen vacancy and polar planes of ZnO. It is difficult to quantificationally determine the correlation. However, a qualitative explanation is that the ZnO polar planes stabilized by OH groups are subject to forming oxygen vacancies by removing either OH or H2O groups from the surface.37,38 Consequently, the ZnO samples with large polar planes generally contain more oxygen vacancies. In addition, screw caps have more oxygen vacancies than disks because the secondary growth for generating the screw caps is beneficial to creating oxygen vacancies. Catalytic Properties. The yield and selectivity of Nformylaniline over the various ZnO catalysts, as well as the result of catalyst blank test, which is similar to that reported in ref 27, are listed in Table 1. Screw caps and rods show the

highest and lowest catalytic activity, respectively, and disks is higher than rings in activity. For comparison, ZnO irregular particles with a size of 0.4-1 µm (Figure S1, Supporting Information) show a 90.0% yield that is only higher than rods. Though the difference in yield is not a huge surprise, the results can show the difference in activity of the ZnO samples considering experimental repeatability (yield error < 1%). Interestingly, the activity order is consistent with the quantitative order of oxygen vacancies determined by PL spectra. This seems to suggest a positive relation between catalytic activity and oxygen vacancy of ZnO. Catalysis reaction is a complicated process where the activity of the catalyst is influenced by many factors. However, activity is essentially dependent on the amount of active sites on the catalysts. Oxygen vacancies are vitally important for catalytic performance of oxide catalysts in many reactions. It was considered that the different defect (including oxygen vacancies) density in the ZnO surfaces played a major role for catalytically decomposing maleic anhydride.39 A scanning tunneling microscopy observation showed that oxygen vacancies could activate formic acid to form an adsorbed formate anion (HCOO-).40 This pronounces that oxygen vacancies are catalytically active for the N-formylation reaction using formic acid as the formylation reagent, because formate anion is an important N-formylation

Morphology-Function Relationship of ZnO

J. Phys. Chem. C, Vol. 112, No. 31, 2008 11863 Conclusions We have synthesized ZnO hexagonal rods with different aspect ratios, disks, rings, and screw caps, which contain different fractions of polar planes, by the facile hydrothermal process. It has been shown that the samples with a large fraction of polar planes contain more oxygen vacancies, indicating that oxygen vacancy may be preferably formed on the polar planes of ZnO. Importantly, oxygen vacancies are catalytically active for the N-formylation reaction of aniline with formic acid as formylation reagent. The synthesized ZnO screw caps containing a large amount of oxygen vacancies show the highest catalytic activity. Therefore, the relationship among polar planes, oxygen vacancies, and catalytic activity is well established, i.e., the more polar planes, the more oxygen vacancies, and the more enhanced catalytic activity for N-formylation. The morphology-function relationship may provide support for designing highly effective, environmental-friendly, and biosafe catalysts for N-formylation in the pharmaceutical synthesis. Acknowledgment. This work is supported by the NCET (040219) of China. Financial support from the Australian Research Council is also gratefully acknowledged. Supporting Information Available: SEM image of the ZnO irregular particles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 5. PL spectra of the synthesized ZnO with various morphologies (a), and Guassian fit results of the 410-1000-nm emissions of ZnO rods (b), rings (c), disks (d), and screw caps (e) with the original profiles in black, the Guassian fit overlaid in red, and the green lines showing the fitted peaks.

intermediate.25,41 Therefore, the ZnO samples containing a relatively large amount of oxygen vacancies have a higher catalytic activity for the N-formylation reaction. Furthermore, the comparison of BET surface area of the ZnO catalysts also reveals the importance of oxygen vacancy and polar plane of ZnO for catalyzing the N-formylation reaction. As shown in Table 1, the BET surface area of screw caps (11.9 m2/g) is close to that of rings (12.5 m2/g) and ∼3 times higher than that of disks (4.6 m2/g) and rods (4.5 m2/g). Usually, a high specific surface area has a beneficial effect on activity for catalysts. However, the activity of rings is lower than that of screw caps, and even lower than that of disks. Compared geometrically to caps and disks, ZnO rings are absent of large polar planes, which are believed to have more oxygen vacancies as discussed above. Meanwhile, the rods with a small proportion of polar planes have lower activity than the disks, though the two samples have subequal surface areas. The results indicate that the amount of oxygen vacancy (formed preferably on polar planes) is more important for catalytic activity than surface area. In addition, the selectivity of N-formylaniline in all the tests is kept above 99.4%, and diphenylformamidine is the sole byproduct detected. It is unclear at present whether the selectivity is relative to oxygen vacancy.

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