Various Facet Tunable ZnO Crystals by a Scalable Solvothermal

Oct 16, 2014 - ABSTRACT: The facet-dependent photocatalytic performance of ZnO is still controversial, for the reason that ZnO samples with different ...
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Various Facet Tunable ZnO Crystals by a Scalable Solvothermal Synthesis and Their Facet-Dependent Photocatalytic Activities Mianli Huang, Sunxian Weng, Bo Wang, Jun Hu, Xianzhi Fu, and Ping Liu* State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fuzhou, Fujian 350002, China S Supporting Information *

ABSTRACT: The facet-dependent photocatalytic performance of ZnO is still controversial, for the reason that ZnO samples with different high energy facets are hard to prepare. In this paper, three kinds of ZnO with different facet exposures, including {0001}, {1011̅ }, and {1010̅ } facets, were prepared by a scalable solvothermal method without using any crystal seeds, environmentally harmful chemicals, or severe reaction conditions. The morphologies of ZnO were gradually varied from prism to polyhedron with the increase of ethylene glycol volume fraction. Meanwhile, the predominant exposed facets changed from {1010̅ } to {1011̅ }. Increasing the volume fraction of ethylene glycol in the mixture solution further, the ZnO spheres formed, where the {0001} facet is predominantly exposed. The formation mechanism for different ZnO geometric structures and their facet-dependent photocatalytic performances were explored in detail, which indicates that the photocatalytic performance is dependent on the crystal facet exposures in the order {0001} > {101̅1} > {101̅0}. Efforts were made to explain the facet-dependent photocatalytic activities basing on the related X-ray photoelectron spectroscopy and atomic stacking model of various exposed surfaces.

1. INTRODUCTION ZnO is one of the most significant oxide semiconductors for applications in catalysis,1 gas sensing,2−4 emissions,5−7 energy harvesting,8 and the biomedical field9 with the merits of good thermal stability, low toxicity, biocompatibility, and low cost. Also, ZnO has aroused worldwide research interest owing to its photocatalysis property. The band gap of ZnO is similar to that of TiO2. Therefore, it is regarded as a substitute for TiO2.10 In general, the photocatalytic reaction occurs at the interface. Photocatalysis requires the effective adsorption of reactant molecules/ions on the surface of ZnO photocatalysts. For specific molecules/ions, their adsorption states are intrinsically determined by surface atomic structures of ZnO.11 Therefore, the photocatalytic performance is strongly associated with the electronic structure and the shape of ZnO.12−14 Recently numerous researchers have been focusing on shape and size control for ZnO crystals, such as ZnO microrods,15 ZnO nanobranches,15 taperlike ZnO,16 and polyhedral 50-facet Cu2O microcrystals,17 which are important in analyzing their facet-dependent physical and chemical properties. In past decades, various shapes of ZnO nanoparticles have been prepared successfully. In general synthesis conditions, ZnO nanostructures tend to grow along the c-axis with predominantly low energy {101̅0} nonpolar facets. Compared with the nonpolar facets, the ZnO semipolar {101̅1} or polar {0001} facets usually grow too fast to be observed in the final shape due to the higher surface energy. However, the high energy facets with unique surface structures are usually more active than the low energy facets.18 It has been demonstrated that © 2014 American Chemical Society

high energy polar and semipolar surfaces may have excellent performance in photocatalysis,19 gas sensors,20 or dye sensitized solar cells.21 So far, there have been some existing preparative techniques for ZnO crystals with high energy facets. Under natural conditions, the polar surfaces usually take up a very small fraction or even vanish because of their high surface energy.22 Surface energies of different surfaces can be effectively decreased by the selective adsorption of appropriate molecules.11 Among them, capping agents and additives are most widely used to tailor the exposure of ZnO crystal facets. Capping agents, such as hydrofluoric acid,23 oleic acid,24 citrate,25 and polyvinylpyrrolidone,26 are utilized to cap the high energy facets to lower the surface energy and growth rate of the metal oxides. Unfortunately, a fatal drawback of most capping agents is that they are corrosive or toxic. What is more, it is also found that the reactive surfaces of the photocatalyst sometimes can be passivated and destroyed by capping agents.27 As a result, a variety of other methods, involving crystal assisted processes,10 ionic liquids,28 the high temperature molten salt route,29 and nonhydrolytic aminolysis synthesis19 have also been employed to control ZnO structures. However, these synthesis methods are usually complicated. Also, the ZnO morphology with different high energy facets is difficult to control well, which is unfavorable to investigating Received: July 21, 2014 Revised: October 15, 2014 Published: October 16, 2014 25434

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the facet-dependent photocatalytic performance of ZnO. Therefore, it is still a great challenge to synthesize ZnO crystals with different facets and more accurately research the relationship between photocatalytic performance and facet orientation. Herein, we present a simple scalable solvothermal approach to synthesize shape controlled ZnO nanostructures without the need for crystal seeds, environmentally harmful chemicals, or severe reaction conditions. In this paper, three kinds of ZnO crystallites exposing various kinds of facets, including {0001}, {101̅1}, and {101̅0}, were successfully synthesized. The formation mechanism and the facet-dependent photocatalytic performance of ZnO have been investigated in detail. Moreover, it is highly significant to discuss the discrepancy of surface atomic structures for different facets in depth, because a better understanding of the reason for facet-dependent photocatalytic activities is necessary to prepare nanomaterials with enhanced properties. It is expected that this facile and controllable synthetic method may inspire the synthesis of similar metal oxides with certain active facets for exotic physical and chemical properties.

mbar with Al Kα as the X-ray source (ESCALAB 250, Thermo Fisher Scientific). Photocatalysis Measurement. The photocatalytic activity of the samples with different facet exposures was estimated by photodegradation of Rhodamine B (RhB) with a concentration of 1 × 10−5 M under the illumination of ultraviolet light. For each reaction, a 0.08 g sample was dispersed in 80 mL of RhB solution in a quartz tube, which was surrounded by four UV lamps of 365 nm (4 W, Philips TL/05). Before illumination, the suspension was sonicated for 5 min. The suspension was kept for 1 h with stirring in the dark to establish adsorption and desorption equilibrium. Then the lamps were turned on to start the photodegradation. About 4 mL of suspension was taken out and centrifuged every 20 min. The concentration of the supernatant liquid was analyzed on a UV−vis spectrometer (Varian Cary 50). Electrochemical Measurements. The photocurrent was conducted on the BAS Epsilon electrochemical system with a conventional three-electrode system. Ten milligrams of ZnO powder was added into 0.1 mL of N,N-dimethylformamide. After sonication for 1 h, the slurry was spread on ITO glasses with an area of 0.25 cm2, which served as the working electrode. As the electrolyte solution, 0.2 mol L−1 Na2SO4 was utilized. A 300 W xenon lamp with 365 nm was used as the light source (CHF-XM300, Beijing Changtuo).

2. EXPERIMENTAL SECTION Materials. Zinc acetate (Zn(AC)2·2H2O), hexamethylenetetramine (HMTA), ethylene glycol (EG), Rhodamine B (RhB), and ethanol were of analytical grade without further purification (Sinopharm Chemical Reagent Co.). Throughout this study, deionized water was used. Synthesis of Materials. Types of ZnO with different facet exposures were prepared by the solvothermal modified method as follows: 5.268 g of Zn(AC)2·2H2O and 3.364 g of hexamethylenetetramine (HMTA) were separately added into 40 mL of a mixture solution of ethylene glycol (EG) and water. The mixture solution was put in a 100 mL Teflon-lined stainless steel autoclave and was then transferred to an oven at 95 °C for 24 h. The obtained precipitate was rinsed with deionized water and ethanol thoroughly, and then dried at 80 °C for 24 h for further characterization. In order to remove organic residues remaining on the samples, the materials were sintered at 300 °C for 3 h. At last, the ZnO prism, polyhedron, and sphere powder were successfully fabricated in 0, 40, and 80 vol % EG solvents, respectively. Materials Structural Characterization. Crystal structure measurements of the products were performed by a Bruker D8 Advance X-ray diffractmeter with a Cu Kα target (λ = 0.154 06 nm) at 40 kV and 40 mA. The step width was 0.02° 2θ s−1. Raman spectroscopy (Renishaw Co.) was observed on a confocal microscopic Raman spectrometer with a 50× microscope objective lens for focusing the 532 nm laser beam. The acquisition time of a Raman spectrum was 5 s. Fourier transform infrared (FT-IR) spectroscopy was performed using a BioRad FTS 6000 analyzer, and the samples were prepared as pellets (sample: KBr (wt %) = 1:100). The morphologies were recorded by a field emission scanning electron microscope (FESEM; FEI Nova NanoSEM 230). The microstructures of the samples were characterized by a Tecnai G2F20 S-TWIN transmission electron microscope (TEM; FEI Co.) with a 200 kV emission gun. The BET surface areas were record by a Micrometrics ASAP 2020 analyzer at 77 K after the products were degassed at 100 °C for 4 h in a vacuum. The photocurrent response for the light or dark short circuit was measured via a BAS Epsilon workstation. X-ray photoelectron spectroscopy (XPS) measurements were operated at 1.2 × 10−9

3. RESULTS AND DISCUSSION Three types of ZnO crystals with different morphologies were synthesized from zinc acetate dihydrate with the assistance of ethylene glycol and H2O of different volume fractions. Figure 1

Figure 1. XRD patterns of different samples.

exhibits the X-ray diffraction (XRD) patterns of all ZnO samples with different facet exposures, which can be clearly indexed to the wurtzite ZnO crystal structure (space group P63mc, a = 3.249 Å and c = 5.206 Å) (JCPDS Card No. 361451).30 No peaks of other phases are observed in the products, which indicate that ZnO is the only phase in the samples. Raman scattering was used to investigate the crystallization and microstructure of the products. The Raman spectra of the samples excited by the 532 nm laser line are illustrated in the Supporting Information, Figure S1. There are two high intensity bands at 99.7 and 441.02 cm−1, which can be assigned to the low and high E2 modes of nonpolar optical phonons, respectively.1 The two weaker bands at 332.7 and 383.11 cm−1 are attributed to the E2H−E2L multiphonons and A1T modes, respectively.4,31 All observed spectroscopic bands can belong to 25435

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hexagonal prism to sphere, as shown in Figure 2i,j. The spherical particles are almost compact and are made of wedgeshaped parts (Figure 2k). The HRTEM image in Figure 2l demonstrates that ZnO wedge-shaped parts are single crystals. The measured fringe spacing of the observed samples are consistent with the lattice spacing of the wurtzite ZnO {101̅0} facets (0.28 nm).40 The corresponding selected area electron diffraction (SAED) pattern indicates that the crystal is aligned radially along the [002̅ ] direction, indicating that the plane of the nanoparticle is {0001} facets. The above structural analysis indicates that three kinds of ZnO products with various facet exposures are prepared via different volume fractions of ethylene glycol and H2O. In detail, the ZnO hexagonal prisms possess the nonpolar {101̅0} and polar {0001} planes. However, for ZnO polyhedron, the predominant facets are semipolar {101̅1} and polar {0001} facets. The predominant facets of ZnO sphere are polar {0001} facets. The plausible formation mechanism of ZnO crystals using different solvents is shown in Figure 3. When only water is used

a typical wurzite ZnO structure, and are similar to the literature values.32 This is in good agreement with the X-ray diffraction data. In order to detect whether any organics remained on the surface of ZnO synthesized with different solvents, a Fourier transform infrared (FT-IR) spectroscopic investigation is displayed in the Supporting Information, Figure S2. The samples of ZnO sphere and polyhedron have almost the same characteristic peaks compared to ZnO prism, which indicates that the residual ethylene glycol can be effectively removed from the surfaces of all samples in the current situation. Typical SEM and high resolution TEM (HRTEM) images of the samples with different facet exposures are illustrated in Figure 2. Figure 2a,b shows the representative FESEM images

Figure 2. SEM and TEM images of different samples: (a−d) ZnO prism, (e−h) ZnO polyhedron, and (i−l) ZnO sphere.

of the ZnO crystals synthesized with the solvent of H2O, which clearly display that they are hexagonal prisms. The HRTEM images (Figure 2c,d) show two-dimensional lattice fringes of 0.28 nm, which could be indexed to {101̅0} crystal facets of wurtzite ZnO, and their side planes are nonpolar {1010̅ } surfaces.33 The inset of Figure 2d is the fast Fourier transform (FFT) pattern, corresponding to the ZnO diffraction pattern of the [0001] zone axis.34 In fact, the crystal preferred orientation along the [0001] direction is due to the crystallographic attribute inherently.35 The concentration of ethylene glycol is crucial in the preparation of different shapes of ZnO. By controlling the volume fraction of ethylene glycol, the size and shape of the ZnO samples were varied. Polyhedron ZnO powder in 40 vol % EG solvents was successfully fabricated, as shown in Figure 2e,f. As labeled in Figure 2f, the angle between the edge and the basal surface is 62°, which is in accordance with the hexagonal cone structure model (62 ± 5°).21,28,36,37 Figure 2h shows the HRTEM image of the truncated ZnO cone with lattice fringes of 0.16 nm, corresponding to the wurtzite ZnO(110) crystal planes. The inset of Figure 2h is the FFT pattern of truncated ZnO cone, which indicates that the top direction of the cone is along the [0001] direction. On the basis of previous research,28,38,39 we conclude that the polyhedron is enclosed by {0001} facets (top and bottom surfaces) and the side surfaces of the cone are the {101̅1} surfaces (inclined surfaces). With further increasing in the volume fraction of ethylene glycol in the mixture solution, the shape changes slowly from

Figure 3. Plausible formation mechanisms of ZnO crystals using different solvents.

as the solvent, tiny nuclei are produced in a high supersaturation. HMTA gradually hydrolyzes to NH3 and HCHO in water, as demonstrated in eqs 1−5.41 In general, wurtzite ZnO grows fast along the polar facets inherently. On the other hand, HMTA is prone to attach to the nonpolar facets promoting the anisotropic growth along the [0001] direction, too. Then ZnO prisms or rods are formed.42 HMTA + 6H 2O ⇄ 4NH3 + 6HCHO

(1)

NH3 + H 2O ⇄ NH4 + + OH−

(2)

Zn

2+

2+

+ 4NH3 ⇄ [Zn(NH3)4 ]

(3)

Zn 2 + + 2OH− ⇄ Zn(OH)2

(4)

Zn(OH)2 ⇄ ZnO + H 2O

(5)

Ethylene glycol served as the neutral solvent and capping material with two OH bonds, which may complex with the Zn2+ ions to form coordination complexes of EG(Zn2+).43,44 Therefore, the nucleating speed can be controlled, which is beneficial for the formation of small nanosheets. Generally, the larger ZnO crystal is a polar crystal whose polar surfaces are rich in positive Zn and negative O, respectively. As a result, the 25436

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positive Zn-rich (0001) facet is the more active plane, which can adsorb ions with the opposite charge or new ZnO species on the plane. Then these tiny nuclei aggregate due to the electrostatic effects of the polar charges on their tops and bottoms.45 Then the self-assembly truncated rhombohedral bipyramid ZnO was obtained. The physical chemistry features of the solvents, such as coordination and viscosity, could affect the solubility, reaction rate, and diffusion motion of the reactants and the intermediate,46 as well as the crystal growth of ZnO. When the volume fraction of EG in the mixture solution increases, the complex formation probability increases, too. The formed plates may experience an attachment to form wedge-shaped parts due to the capping ability of EG. On the other hand, the viscosity of EG is much higher than that of H2O. The ion diffusion rate is slower in EG than in H2O. The aggregates are prone to fuse to each other to minimize their total surface energy. Eventually, the plates self-assemble to form microspheres. Successful preparation of ZnO types with different exposed surfaces is beneficial to systematically research the facetdependent photocatalytic performance of ZnO crystals. The UV−vis absorption spectra of three kinds of ZnO products are displayed in the Supporting Information, Figure S3. It is clear that the light absorption properties of the as-prepared three kinds of ZnO samples are almost unchanged. In this study, photodegradation of Rhodamine B (RhB) as a model organic pollutant with a concentration of 10 × 10−5 M was carried out under UV illumination. The UV−vis adsorption spectra of RhB solution as a function of illumination time are given in the Supporting Information, Figure S4. There is a major absorption peak at 554 nm for RhB. Both negligible wavelength shifts and absorption peak intensity can be observed under illumination, which indicates that the RhB solution is quite stable to illumination. By contrast, when the three kinds of ZnO photocatalysts are added, the 554 nm adsorption peak is decreased greatly with irradiation time. Meanwhile, no new absorption peaks are observed confirming the mineralization of RhB.47 Prior to illumination, the suspension was kept for 60 min with stirring in the dark for adsorption−desorption equilibrium. Figure 4a displays the concentration changes of RhB during the degradation process. After UV illumination for 2 h, approximately 58% of RhB is photodegraded by the sample of ZnO prism. As for ZnO polyhedron, approximately 84% of RhB is decomposed, and the ratio reaches 96% for ZnO sphere samples. The photocatalytic degradation process follows pseudo-first-order kinetics by linear transforms ln(C0/Ct) = kt, where k is the apparent first-order rate constant. As confirmed in Figure 4b, the rate constants (k) of the products ranged from 0.0069 to 0.025 min−1, decreasing in the order ZnO sphere > ZnO polyhedron > ZnO prism. BET surface area is one of the most crucial influences on photocatalytic performance. From Table 1, it is noted that the discrepancy of BET surface area for different samples is very small. As a result, the rate constant of ZnO sphere normalized by the surface area (k′) is also higher than those of ZnO polyhedron and ZnO prism (Figure 5), which suggests that there are other factors influencing the photocatalytic efficiency besides the special surface area in our system. Considering the samples with different facet exposures, facet orientation may be the main factor influencing the photocatalytic performance. The ratios of predominant facets for different ZnO samples are also displayed in Table 1. The ZnO sphere with the

Figure 4. (a) Photocatalytic degradation of RhB under UV light illumination. (b) Linear fitting results of pseudo-first-order kinetics.

predominant facet of {0001} shows the highest photocatalytic activity. We could conclude that the large ratio of {0001} crystal facets is in favor of enhancing photocatalytic performance. The ZnO polyhedral particle has not only the {0001} facet, but also 12 {1011̅ } facets. The basal and pyramidal planes are identified as {0001} polar and {101̅1} semipolar surfaces, respectively. The photocatalytic efficiency of the polyhedron is therefore slightly lower than that of the ZnO sphere. Also, the ZnO prism consists of {1010̅ } and {0001} facets. Based on detailed analyses of many nanoscale prisms, the surface areas of polar {0001} and nonpolar {101̅0} facets are estimated to be 38.78 and 61.22%, respectively. Compared with the ZnO polyhedral, the ratio of active {0001} facet is higher. However, the ZnO prism exhibits the lowest photocatalytic efficiency. As a consequence, we can conclude that the order of photodegradation efficiencies for different surfaces of ZnO should be {0001} > {1011̅ } > {1010̅ }, as estimated by degeneration of RhB. The variations of the fast and uniform photocurrent response under intermittent 365 nm UV light illumination for each switch-on and switch-off event are recorded in Figure 6.48 The result indicates that the order of the photocurrent is in accordance with the order of the rate constant (k). When the light is turned on, photogenerated electrons diffuse to the ITO glasses. Then the photocurrent is formed. The enhanced photocurrent of the sample ZnO sphere indicated an enhanced separation efficiency of photoinduced electrons and holes.49 When the illumination is stopped, the photocurrent decreases back to zero because the photoinduced electrons may be captured by the electron acceptors quickly in the dark.50 XPS analyses are used to investigate the surface structures of all samples. As shown in Figure 7a, Zn 2p XPS spectra of ZnO prism, polyhedron, and sphere are analogous for their location and distribution. The O 1s spectra can be fitted into two bands, shown in Figure 7b−d. The bands at about 530 eV are related to the ZnO crystal lattice oxygen (OL), which corresponds to 25437

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Table 1. Physicochemical Properties of Different ZnO Samples ratio of dominant facets (%) sample ZnO prism ZnO polyhedron ZnO sphere

{101̅0} {101̅1}

61.22 76.53

{0001} {0001} {0001}

BET surf. area (m2g−1)

k (min−1)

k′ (g min−1 m−2)

15.45 20.15 12.91

0.0069 0.015 0.025

0.000 45 0.000 76 0.001 9

38.78 23.47 100

Table 2. Curve-Fitting Results of O 1s XPS Spectra ZnO sample ZnO prism

ZnO polyhedron

ZnO sphere

Figure 5. Comparison of k (left axis) and k′ (right axis) for photocatalytic decomposing RhB under UV light illumination.

binding energy (eV) relative percentage (%) binding energy (eV) relative percentage (%) binding energy (eV) relative percentage (%)

lattice oxygen (OL)

chemisorbed oxygen (OC)

530.0

531.33

70.4

29.6

530.04

531.34

64.1

35.9

530.05

531.35

62.1

37.9

of the chemisorbed oxygen (OC) in three kinds of ZnO are 29.6% (prism), 35.9% (polyhedron), and 37.9% (sphere), respectively. This indicates that the ZnO sphere may attract more oxygen species or OH. The adsorption ability of ZnO polyhedron is worse. As for ZnO prism, the adsorption ability is the worst. Surface hydroxyl is an important active species of semiconductor photocatalysis.53 Therefore, the photocatalytic activity of ZnO samples would increase as the relative percentages of the chemisorbed oxygen species OC or OH component increased. The difference in surface atomic structures may lead to the discrepancy in the ability of chemisorbed oxygen species or OH. Therefore, the wurtzite ZnO atomic stacking model is analyzed in Figure 8a, which shows the atom arrangements in Figure 6. Photocurrent responses of different ZnO samples in 0.2 M Na2SO4 aqueous solutions under UV light illumination.

Figure 8. (a) ZnO atomic stacking model, including {0001}, {101̅1}, and {1010̅ } facets. (b−d) Schematic illustrations of ZnO prism, polyhedron, and sphere, respectively.

{0001}, {1011̅ }, and {1010̅ } surfaces. The morphological schematic illustrations of the as-prepared ZnO samples are shown in Figure 8b-d. ZnO is the typical polar crystal with the positively Zn2+-terminated {0001} and negatively O2−-terminated {0001}̅ polar surfaces.54 Zn2+ ions on the polar facets can capture oxygen species via chemical or physical absorption because of unsaturated oxygen coordination.34 ZnO spheres expose the largest ratio of {0001} surface; thus their

Figure 7. (a) XPS spectra of Zn 2p. (b−d) O 1s XPS spectra of ZnO prism, polyhedron, and sphere, respectively.

the O−Zn bond.51 According to earlier literature,52 the band at about 531 eV is due to surface chemisorbed oxygen species (OC) or OH. As displayed in Table 2, the relative percentages 25438

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(3) Xu, J.; Pan, Q.; Shun, Y. a.; Tian, Z. Grain Size Control and Gas Sensing Properties of ZnO Gas Sensor. Sens. Actuators, B 2000, 66, 277−279. (4) Zhang, D.; Wu, X.; Han, N.; Chen, Y. Chemical Vapor Deposition Preparation of Nanostructured ZnO Particles and Their Gas-Sensing Properties. J. Nanopart. Res. 2013, 15, 1580−1590. (5) Tsukazaki, A.; Ohtomo, A.; Onuma, T.; Ohtani, M.; Makino, T.; Sumiya, M.; Ohtani, K.; Chichibu, S. F.; Fuke, S.; Segawa, Y.; et al. Repeated Temperature Modulation Epitaxy for p-Type Doping and Light-Emitting Diode Based on ZnO. Nat. Mater. 2004, 4, 42−46. (6) Subramanian, V.; Wolf, E. E.; Kamat, P. V. Green Emission to Probe Photoinduced Charging Events in ZnO−Au Nanoparticles. Charge Distribution and Fermi-Level Equilibration. J. Phys. Chem. B 2003, 107, 7479−7485. (7) Illyaskutty, N.; Sreedhar, S.; Kohler, H.; Philip, R.; Rajan, V.; Pillai, V. P. M. ZnO-Modified MoO3 Nano-Rods, -Wires, -Belts and -Tubes: Photophysical and Nonlinear Optical Properties. J. Phys. Chem. C 2013, 117, 7818−7829. (8) Chen, X.; Xu, S.; Yao, N.; Shi, Y. 1.6 V Nanogenerator for Mechanical Energy Harvesting Using PZT Nanofibers. Nano Lett. 2010, 10, 2133−2137. (9) Xu, X.; Chen, D.; Yi, Z.; Jiang, M.; Wang, L.; Zhou, Z.; Fan, X.; Wang, Y.; Hui, D. Antimicrobial Mechanism Based on H2O2 Generation at Oxygen Vacancies in ZnO Crystals. Langmuir 2013, 29, 5573−5580. (10) Huang, M.; Yan, Y.; Feng, W.; Weng, S.; Zheng, Z.; Fu, X.; Liu, P. Controllable Tuning Various Ratios of ZnO Polar Facets by Crystal Seed-Assisted Growth and Their Photocatalytic Activity. Cryst. Growth Des. 2014, 14, 2179−2186. (11) Liu, G.; Yang, H. G.; Pan, J.; Yang, Y. Q.; Lu, G. Q.; Cheng, H. M. Titanium Dioxide Crystals with Tailored Facets. Chem. Rev. 2014, 114, 9559−9612. (12) Boppella, R.; Anjaneyulu, K.; Basak, P.; Manorama, S. V. Facile Synthesis of Face Oriented ZnO Crystals: Tunable Polar Facets and Shape Induced Enhanced Photocatalytic Performance. J. Phys. Chem. C 2013, 117, 4597−4605. (13) Alenezi, M. R.; Alshammari, A. S.; Jayawardena, K. D. G. I.; Beliatis, M. J.; Henley, S. J.; Silva, S. R. P. Role of the Exposed Polar Facets in the Performance of Thermally and UV Activated ZnO Nanostructured Gas Sensors. J. Phys. Chem. C 2013, 117, 17850− 17858. (14) Xiao, Y.; Lu, L.; Zhang, A.; Zhang, Y.; Sun, L.; Huo, L.; Li, F. Highly Enhanced Acetone Sensing Performances of Porous and Single Crystalline ZnO Nanosheets: High Percentage of Exposed (100) Facets Working Together with Surface Modification with Pd Nanoparticles. ACS Appl. Mater. Interfaces 2012, 4, 3797−3804. (15) Zhang, T.; Dong, W.; Njabon, R. N.; Varadan, V. K.; Tian, Z. R. Kinetically Probing Site-Specific Heterogeneous Nucleation and Hierarchical Growth of Nanobranches. J. Phys. Chem. C 2007, 111, 13691−13695. (16) Zhang, T.; Dong, W.; Keeter-Brewer, M.; Konar, S.; Njabon, R. N.; Tian, Z. R. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses of Branched ZnO Crystallites. J. Am. Chem. Soc. 2006, 128, 10960−10968. (17) Liang, Y.; Shang, L.; Bian, T.; Zhou, C.; Zhang, D.; Yu, H.; Xu, H.; Shi, Z.; Zhang, T.; Wu, L. Z.; et al. Shape-Controlled Synthesis of Polyhedral 50-Facet Cu2O Microcrystals with High-Index Facets. CrystEngComm 2012, 14, 4431−4436. (18) Huang, X.; Zhao, Z.; Fan, J.; Tan, Y.; Zheng, N. Amine-Assisted Synthesis of Concave Polyhedral Platinum Nanocrystals Having {411} High-Index Facets. J. Am. Chem. Soc. 2011, 133, 4718−4721. (19) Chang, J.; Waclawik, E. R. Facet-Controlled Self-Assembly of ZnO Nanocrystals by Non-Hydrolytic Aminolysis and Their Photodegradation Activities. CrystEngComm 2012, 14, 4041−4048. (20) Liu, J.; Chen, X.; Wang, W.; Liu, Y.; Huang, Q.; Guo, Z. SelfAssembly of [101̅0] Grown ZnO Nanowhiskers with Exposed Reactive (0001) Facets on Hollow Spheres and Their Enhanced Gas Sensitivity. CrystEngComm 2011, 13, 3425−3431.

photocatalytic properties are markedly enhanced. For ZnO polyhedron with truncated double-cone structures, Zn-terminated and O-terminated {101̅1} facets coexist on opposite sides.55 The ZnO {101̅1} facet along the positive axis is covered exclusively by Zn2+ ions. Also, the opposing facet contains only O2− ions, resulting in opposite polarities.21 The sample shows medium ability of chemisorbed oxygen species. However, the predominant facets of ZnO prism are the nonpolar {101̅0} facets with equivalent Zn and O atoms. The ability of absorbing oxygen species is quite poor. Therefore, its photocatalytic property is the worst.

4. CONCLUSION In summary, ZnO structures with different facet exposures, including {0001}, {101̅1}, and {101̅0} facets, were prepared by a scalable solvothermal method without using crystal seeds, environmentally harmful chemicals, or severe reaction conditions. The concentration of ethylene glycol is crucial to the growth property of ZnO crystal. The morphologies of ZnO were gradually varied from prism to polyhedron with the increase of ethylene glycol volume fraction. The predominant exposed facets varied from {101̅0} to {101̅1}. Increasing the volume fraction of ethylene glycol in the mixture solution further, the ZnO spheres formed, where the {0001} facet is predominantly exposed. The facet-dependent photocatalytic performance indicates that the photocatalytic performance is dependent on the crystal facet exposures in the order {0001} > {101̅1} > {101̅0}, which is in accordance with the ability of chemisorbed oxygen species or OH. It is expected that this facile and controllable synthetic method may inspire the synthesis of similar metal oxides with certain active facets for exotic physical and chemical properties.

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ASSOCIATED CONTENT

S Supporting Information *

Raman spectra, Fourier transform infrared spectra, UV−vis absorption spectra, temporal UV−visible adsorption spectral changes, and references. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail:[email protected]. Fax: +86-591- 83779239. Tel.: +86-591- 83779239. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21173046, 21273035, and 21473031) and the National Basic Research Program of China (2013CB632405 and 2014CB239303).

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp5072567 | J. Phys. Chem. C 2014, 118, 25434−25440