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Self-assembled single-crystalline ZnO nanostructures3 Cite this: CrystEngComm, 2013, 15, 3780 Received 25th January 2013, Accepted 27th March 2013
Lifang Chen,a Juncheng Hu,b Feng Lin,c Christopher Cadigan,b Wenrong Cao,a Zhiwen Qi,*a Marta Pozuelo,d Sergey V. Prikhodko,d Suneel Kodambakad and Ryan M. Richards*c
DOI: 10.1039/c3ce40167g www.rsc.org/crystengcomm
We report a template-free, halide-free, efficient wet chemical method to synthesize defect-rich ZnO nanostructures with ¯ 1} facets. The self-assembled ZnO nanostructures exposed {101 provide an active playground for catalytic reactions, such as CO2 hydrogenation, and exhibits great potential in alternative energy technologies.
The controlled synthesis and characterization of low dimensional crystalline nano/micro materials is a major objective in modern materials science, physics, and chemistry.1 Many researchers have focused on rational ways to control the shape, size, and dimensionality of nanomaterials. Self-assembly of inorganic nano building blocks into one-, two-, and three-dimensional ordered hierarchical nanostructures is fascinating because the variation in arrangements of the building blocks provides a method to tune the properties of the material.2 ZnO is particularly interesting as an excellent optoelectronic material because of its wide direct band gap and high exciton binding energy. ZnO has been demonstrating interesting properties with a wide variety of morphologies, such as nanowires, nanobelts, cubes, sheets, and tetrapods. Many efforts have been exerted to prepare ZnO possessing controlled shapes and morphologies by different growth mechanisms under a wide range of synthesis conditions.3 The interest in fabricating new ZnO nanostructures with different morphologies arises from the fact that ZnO is highly anisotropic, i.e. its physical and chemical properties vary with surface facet ¯0} facets are the orientations.4–6 In wurtzite-structured ZnO, {101 lowest energy surfaces and non-polar, {0001} surfaces are polar ¯1} facets are always with either Zn- or O-terminations, and {101 a
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China. E-mail:
[email protected] b Key Lab of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan 430074, China c Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado, 80401, USA. E-mail:
[email protected] d Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, CA 90095, USA 3 Electronic supplementary information (ESI) available: Experimental method, additional TEM images and XRD patterns. See DOI: 10.1039/c3ce40167g
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O-terminated.5d,6 While the majority of the synthesized ZnO ¯0} facets in order to minimize nanostructures are bounded by {101 surface energy under prepared processes, a few papers have also reported the growth of ZnO crystals with {0001} surfaces exposed.5 ¯1} facets have higher Since the O-terminated {0001} and {101 surface activity, controlled preparation of micro/nano ZnO crystals ¯1} bounded predominantly by O-terminated (0001) and {101 surfaces are highly desirable for catalytic studies. Carbon dioxide is the major greenhouse gas, and its atmospheric concentration continues to increase from burning fossilfuels.7 Conversion of carbon dioxide to higher value molecules is challenging due to its stability. Materials capable of functionalizing it may provide insight into new paradigms for upgrading. Carbon dioxide hydrogenation is generally conducted with ZnO supported metallic Cu-containing binary catalysts at high pressures.8 However, catalytic activity in pure ZnO (particularly at atmospheric pressure) has not yet been identified experimentally.9 Although a few single-crystalline ZnO nanostructures with 6-fold building blocks have been reported previously, these synthesized ¯0} facets.10 The ZnO building blocks are bounded by nonpolar {101 synthesis of ZnO nanostructures with 6-fold building blocks ¯1} facets has been rarely exploited enclosed by higher energy {101 and is an important challenge. Herein, we describe a template-free, efficient solvent-thermal method combined with pseudo-supercritical fluid drying (methanol critical point: 240 uC with 79.9 bar) to synthesize ZnO nanostructures, which are composed of 6-fold building blocks ¯1} facets, using zinc nitrate as the bounded by large amount of {101 starting material, and excellent yields and high crystallinity of the products. The Cu-free ZnO composed of 6-fold building blocks ¯1} surfaces, shows surpriswith high percentages of exposed {101 ing activity for CO2 hydrogenation and exhibits great potential in alternative energy technologies. The morphology of the ZnO nanostructures is shown in Fig. 1a–c by field-emission scanning electron microscopy (FESEM) at different magnifications. It is observed that the ZnO product contains numerous aggregates in Fig. 1a. These flowers are composed of 6-fold building blocks, as shown in Fig. 1b. The diameters of these 6-fold building blocks are ca. 0.5–3 mm, and the
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Fig. 1 (a)–(c) FESEM images of self-assembled ZnO nanostructures at different magnifications; (d) XRD patterns of ZnO nanostructures and standard JCPDS 361451.
lengths are ca. 2–6 mm. It is notable that many holes are also observed in these 6-fold building blocks owing to removal of the pore fluid by supercritical drying in Fig. 1c. Fig. 1d shows the powder X-ray diffraction (XRD) pattern of ZnO nanostructures calcined at 500 uC. The sample was synthesized when these reactants were in the ratio of Zn(NO3)2?6H2O : urea : benzyl alcohol = 1 : 0.5 : 2 (molar ratio). The diffraction peaks at 2h = 31.7u, 34.4u, 36.3u, 47.5u, 56.6u, 62.8u, 66.4u, 67.9u, 69.1u correspond to (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes, respectively of wurtzite-structured (hexagonal) ZnO (JCPDS card no. 36-1451; space group: P63mc(186)) with lattice constants a = 0.325 nm and c = 0.521 nm. The morphology and structure of these 6-folded building blocks were further confirmed by TEM, high resolution TEM (HRTEM), and selected area electron diffraction (SAED) in Fig. 2. Fig. 2a reveals the morphology of 6-fold building blocks with holey structure. Fig. 2b shows the associated SAED pattern oriented to ¯0] zone axis, and it can be indexed as single-crystalline ZnO [011 with a hexagonal structure. The HRTEM image, obtained from the edge of the selected 6-fold building block, clearly shows lattice fringes with an interplanar spacing of 0.26 nm that corresponds to the distance between (0002) crystal planes. The inset is a Fourier transform (FT) of the HRTEM image, indicating hexagonal structure and is consistent with the SAED pattern in Fig. 2b. From the SEM and TEM images, along with the SAED data, the exposed surfaces of the 6-fold building block can be visualized as a hexagonal prism with a pyramidal tip as shown in Fig. 2d. From the geometry, we infer that the facets bounding the pyramidal tip ¯1} planes and the tip growth is along the [0001] direction are {101 and the angle a between the tip facets and the base is 61.39 degrees. Due to the anisotropic structure of ZnO, growth along polar [0001] direction is likely to be faster resulting in the observed morphology. This is consistent with previous studies, which suggested that the orientation-dependent growth velocities V are ¯0] . V[1000] under hydrothermal such that V[0001] . V[011 condition.11
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Fig. 2 (a) Transmission electron microscopy (TEM) image of 6-folded nano building blocks; (b) characteristic selected area electron diffraction (SAED) ¯ 0] zone axis; (c) pattern of an individual 6-folded building block indexed in [011 high-resolution TEM image of the region highlighted by a rectangle in (a). Inset is Fourier transform of the image; (d) schematic diagrams of the ZnO building block (top) and the ZnO tip (bottom). The apex angle BAC subtended by the lines BA and AC is 64.42 degrees and the angle ODA is referred to as a in the text.
Fig. 3 shows the schematic models of the atomic structures on ¯0}, {0001}, and {101 ¯1} crystal surfaces of wurtzite ZnO, {101 ¯0} surfaces respectively. Among these low-index surfaces, the {101 are non-polar, i.e. electrically neutral, have relatively low surface energy compared to the other surfaces, and hence are energetically ¯1} surfaces have stable. In contrast, the polar {0001} and {101 higher surface energies. The {0001} surface of ZnO is Zn or O ¯1} is always O terminated as determined terminated, whereas {101 via dynamic laser ablation under a specified oxygen flow rate.6c ¯} surface, which is Only the O22 anions are accessible on the {0001 responsible for the surface activity and is a single oxygen atom ¯1} surface is always O-terminated and has layer while the {101 ¯1} surfaces with the double O-atom layers.6b,12 Therefore, the {101 highest surface energy are less likely to appear on the bare surfaces during crystal growth. In our preparation process, the diameters of these 6-fold building blocks are wide, while the lengths are relatively short, indicative of the large amount of tips with polar ¯1} surfaces (.75%) as observed in Fig. 1a. These exposed {101 ¯1} surfaces need to eliminate surplus charges to form the {101 ¯} surfaces. Therefore, it is stable surface being similar to {0001 plausible that large amounts of oxygen vacancies form on/near the ¯1} surfaces, and give rise to special semiconducting, and/or {101 catalytic applications.6b,12b In order to determine the formation mechanism and to identify the critical parameters controlling the shapes and sizes of
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Fig. 3 Ball and sticks model representation of wurtzite ZnO surfaces with (a) ¯0}, (b) {0001 ¯}, and (c) {101 ¯1} direction. The red and green colored spheres {101 represent O and Zn atoms, respectively.
these ZnO nanostructures, several experiments were carried out. In the absence of benzyl alcohol and urea, ZnO with irregular shapes were obtained as shown in Fig. SI1, ESI.3 At the constant ratio of Zn(NO3)2?6H2O : benzyl alcohol (1 : 2, molar ratio), the size of ZnO 6-folded building blocks decreases with decreasing amounts of urea as shown in Fig. SI2, ESI.3 Smaller ZnO 6-folded building blocks with diameter ca. 1 mm were obtained when the ratio of Zn(NO3)2?6H2O : urea = 1 : 0.25, and the aggregates resemble a blooming flower as shown in Fig. SI2a, ESI.3 In the absence of urea, the length and diameter of ZnO 6-folded building blocks decrease considerably and the aggregates of these building blocks appear like a flower bud as shown in Fig. SI2b, ESI.3 Based upon the experimental results, the role of urea is important to control the size of the ZnO 6-folded building blocks and the shape of aggregates in the synthesis method as it provides a steady OH2 supply via urea hydrolysis.13 When Zn(NO3)2?6H2O reacts with methanol and water to form the ZnO precursor, acid is a byproduct, and the accumulation of acid will inhibit the further formation of the ZnO precursor. However, when urea is added, the OH2 formed by urea hydrolysis neutralizes the acid and allows the formation of the ZnO precursor. Thus, in the self-assembly process, we suggest that the steady OH2 supply controls the size of ZnO building blocks by moderating the hydrolysis and alcoholysis rates of zinc nitrate. Benzyl alcohol has been found to be a successful medium for tailoring selected metal oxides with well-controlled shapes, sizes, and crystallinity under anhydrous conditions.2d Here, benzyl
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CrystEngComm alcohol is used as a structure-directing agent to control the synthesis of ZnO nanostructures. In the absence of benzyl alcohol, although 6-fold building blocks or prisms were obtained; the length and diameter are inhomogeneous (Fig. SI3a, ESI3). Layered columns (Fig. SI3b, ESI3) were formed with the molar ratio of Zn(NO3)2?6H2O : urea : benzyl alcohol = 1 : 0.5 : 6. In the literature, most hypotheses regarding the role of organic compounds is that they function as simple physical compartments or act to control nucleation or to terminate crystal growth by surface poisoning through selective adsorption on specific planes.14 In this work, a large amount of benzyl alcohol leads to reduced length of 6-fold ZnO building blocks, and forms these layered ZnO columns. This infers that benzyl alcohol plays a critical role as a structure-directing agent to control the shape of these 6-fold building blocks, and may be adsorbed on (0002) planes to terminate the crystal growth. There are many holes in all these ZnO samples prepared by different methods. Wang et al. reported the preparation of ZnO rings from ZnO disks and attributed the formed hole to a high density of defects at the center of the ZnO disks that resulted in a high local reaction/etching rate under heating.15 In this case, a similar mechanism may occur with the high reaction/etching rates at the defect sites in the ZnO building blocks leading to the formation of holes. In addition, XRD results showed that all ZnO samples here are single phase of wellcrystallized ZnO with the hexagonal wurtzite structure (Fig. SI4, ESI3). Atom stacking on metal oxide surfaces significantly influence a variety of surface properties, including surface activity.16 The ¯1} facets exhibit high catalytic activity, and are likely to have {101 many oxygen vacancies, which are also of particular importance, leading to deviation in the zinc-to-oxygen ratio.17 In this work, we prepared single-crystalline ZnO with a large amount of holes and ¯1} surfaces, meaning high density of defects exposed polar {101 with highly active sites on the surfaces. Controlled shapes and morphologies of ZnO nanostructures have been fabricated successfully. However, their properties, particularly catalytic activities of the prepared ZnO nanostructures have not been reported. As carbon dioxide is a major greenhouse gas and a cheap C1 resource, it has drawn wide attention in recent years to convert CO2 into value-added chemicals,18 and the hydrogenation of CO2 to produce C–H compounds is particularly attractive.7,19 CO2 hydrogenation is predominantly accomplished on Cu/ZnO catalysts system at high pressures.8,9 Here, CO2/H2 adsorption and reaction on the surface of ZnO nanostructures in Fig. 4a have been performed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic techniques at low temperatures and atmospheric pressure. Surprisingly, the Cu-free ZnO exhibited high activity for CO2 hydrogenation as shown in Fig. 4a. CO2 was not adsorbed on the surface of single crystalline ZnO at 180 uC for 1 min (Fig. 4a, trace 1) likely because of its low surface area (2 m2 g21). However, after heating at 180 uC for 10 min, CLO stretching bands at 1740 cm21 were observed, which can be attributed to adsorbed formate and the band at 1711 cm21 is assigned to adsorbed Z–H species;20 the bands at 1516, 1368, 1214 cm21 are due to adsorbed formic acid and the band at 1048 cm21
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Communication A comparison of CO2 adsorption behaviour between single crystalline ZnO and NanoActive ZnO is presented as an inset in Fig. 4b. NanoActive ZnO has comparatively favorable surface area with that of single crystalline ZnO, which can be confirmed by the relatively weak diffraction of XRD and TEM image with small particle sizes in Fig. SI5a and SI6, ESI.3 The HRTEM image of NanoActive ZnO in Fig. SI7, ESI3 shows lattice fringes of 0.26 nm corresponding to the distance between (0002) crystal planes with exposed low surface energy facets (see Fig. 2d, the bulk of the building block, and Fig. 3c). Moreover, the strengths of C–H, CLO, and C–O stretching bands for single crystalline ZnO with low surface area increase with increased reaction time suggesting higher surface activity of the single crystalline ZnO than that of NanoActive ZnO with higher surface area. We infer that the higher ¯1} surface activity is due to the presence of the exposed polar {101 facets.
Conclusion
Fig. 4 DRIFT spectra obtained after exposing (a) the single crystalline ZnO and (b) NanoActive ZnO to CO2 and H2 at 180 uC for (1) 1 min, (2) 10 min, (3) 20 min, (4) 40 min, (5) 60 min, and the inset in (b) is a reference bar of (1) between single crystalline ZnO and NanoActive ZnO.
In summary, we have experimentally demonstrated the direct synthesis of single crystalline ZnO nanostructures composed of ¯1} facets of the the 6-folded building blocks with exposed {101 highest surface energy among the low-index surfaces containing a large number of holes. The sizes and shapes of hole-rich single crystalline ZnO can be controlled by varying the amount of urea and benzyl alcohol. It is notable that the single-crystalline, Cu-free ZnO with low surface area (2 m2 g21) showed much higher surface activity for CO2 hydrogenation than high surface area (120 m2 g21) NanoActive ZnO. These results are promising for the use of these ¯1} surfaces as ZnO morphologies with exposed polar {101 heterogeneous catalysts, catalyst supports, and potentially in other applications.
Acknowledgements corresponds to C–O stretching of adsorbed methoxyl group.21 Moreover, the C–H stretching bands were observed at 2973 and 2881 cm21, indicative of CO2 hydrogenation. The results indicate that the hydrogenation of CO2 produces formic acid, formaldehyde, and methanol. In contrast, the hydrogenation of CO2 was not observed over NanoActive ZnO with high surface area (from NanoScale Corporation, 120 m2 g21 and crystallite size ¡5 nm), while this material has demonstrated significantly enhanced reactivity with CCl4, SO2, and paraoxon in comparison to commercial ZnO.22 Absorption was observed in the range of 1000–1700 cm21 after exposing NanoActive ZnO to CO2 and H2 at 180 uC for 1 min (Fig. 4b, trace 1), which may be due to the large surface area. The major bands of adsorbed CO2 are at 1579 and 1687 cm21, which are assigned to bending OCO vibrations of adsorbed formate ions23 or bi-dentate carbonate of CO2 on the basic ZnO.24 In addition, various ill-defined bands associated with carboxylates and carbonates (1399 and 1311 cm21) could be observed in the 1000–1700 cm21 region.23a All bands at these regions did not change appreciably in intensity with increased time. The characteristic behavior of these observed bands makes it clear that NanoActive ZnO could not catalyze CO2 hydrogenation.
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This work was supported by the National Natural Science Foundation of China (NSFC 21006029), National High Technology Research and Development Program of China (863 Program 2012AA0161601), Shanghai Natural Science Foundation (10ZR1407200), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP 20120074110008), the Open Project of State Key Laboratory of Chemical Engineering (SKL-Che-12C05), 111 Project (B08021), the Colorado School of Mines, the American Chemical Society Petroleum Research Foundation (Grant #48108-G10), the NSF through the grant CMMI-1200547, and the National Renewable Energy Laboratory’s Hydrogen Systems and Technologies Center.
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