ZnO tetrakaidecahedrons with co-exposed {001}, {101} and {100

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ZnO tetrakaidecahedrons with co-exposed {001}, {101} and {100} facets: shape-selective synthesis and enhancing photocatalytic performance Yujie Liu, dong Huang, Haixia Liu, Tianduo Li, and Jingui Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01886 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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Crystal Growth & Design

ZnO tetrakaidecahedrons with co-exposed {001}, {101} and {100} facets: shape-selective synthesis and enhancing photocatalytic performance Yujie Liu, Dong Huang, Haixia Liu1*, Tianduo Li, Jingui Wang Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, PR China Abstract The active crystal plane determines the activity of the inorganic metal oxide-based photocatalyst, especially in the degradation of organic pollutants. The ZnO tetrakaidecahedron with different crystal faces ({001}, {101} and {100}) is efficiently synthesized with tetramethylammonium hydroxide (TMAH) as an additive. The exposed surface of ZnO tetrakaidecahedron can be controlled by changing the reaction concentration and reaction time. The tetradecahedral ZnO (ZnO-1, ZnO-2) nanoparticles were systematically investigated by various characterizations. Based on the experimental results, we speculated the possible formation mechanism of ZnO

Yujie Liu and Dong Huang are co-first authors of the article. * Corresponding author.

E-mail address: [email protected] (H. Liu).

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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tetrakaidecahedron. In addition, the photocatalytic activity of ZnO-1 tetrakaidecahedron is better than that of ZnO-2 nanoparticles in photodegradation of methylene blue (MB) and Rhodamine B (Rh B), which ascribed to the more exposed active crystal faces, the large photocurrent density and large specific surface area. 1. Introduction The morphology of metal oxide semiconductor nanocrystals has recently received considerable attention owing to their photocatalysis and optoelectronics performance, which can be improved and stabilized through tailoring the surface structures and the exposed crystal faces. 1-4 Inorganic metal oxide, such as TiO2, BiVO4, and Ag2O, as photoactive materials has been currently under intense research. Liu et al. synthesized a high-activity TiO2 modified with {001} surface by controlling the hydrothermal time.5 While Wee-Jun Ong et al. reviewed main strategies in preparing the highly active {001} face of TiO2-based composites. 6 Xu et al. synthesized a novel promising nanosheet BiVO4 by hydrothermal method which has a reduced size, sufficient oxygen vacancy (OVs) and exposed {001} plane for the removel of oxytetracycline. 7 Moreover, Wang et al. got the Ag2O microcrystals with exposed {100} facets and various morphologies, which displayed the superior photocatalytic performance.

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Unfortunately, highly reactive

surfaces are important for improving photocatalytic reactivity, but due to the minimization of surface energy, they generally decrease rapidly during crystal growth.9-10 To solve this problem, various additives are commonly used to selectively regulate

11the

growth rate of the planes.

Therefore, the surface-regulated fabrication of nanocrystals is not only an intelligent route to study the relations between the facets (morphology) structures and the photocatalytic properties but also a feasible method to develop highly active photocatalysts. ZnO crystal in the wurtzite


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phase is an important semiconducting metal oxide, owing to its abundance, low-cost, nontoxicity, and thermal stability. 12-14 ZnO nanostructures often have interesting physical or chemical properties for optical, catalytic, sensing or antibacterial applications.

15-17

In general, proper control of high active surfaces

(morphologies) and photocatalytic properties of organic pollutant degradation have been a longterm goal. Zhang et al. demonstrated the simple hydrothermal synthesis of high-efficiency photocatalysts composed of ZnO nanosheet clusters by exposing the catalytic interface to the space of the (001) plane.

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More recently, Li et al. found that ZnO double spheres exposed to

(001) polar plane had high potential, which further adjusted the properties of the material and improved its performance in blue luminescent agents and other applications.19 In the presence of Si substrates, single crystal ripple-like ZnO nanobelt were prepared by chemical vapor deposition. And the flat side-facets (100) were close to the high-index surface (0,10,1) and the main surface of this kind of ripple-like structure belongs to (2,1(-),0) plane. 20 Furthermore, some references reported higher photocatalytic activity of ZnO nanocrystals with well-faceted {101} surfaces on organic contaminants.

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Waclawik et al. synthesized the cone-shape ZnO

nanocrystals with {101} active O-terminated surfaces and applied them in dye-sensitized solar cell devices.

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However, to our knowledge, the study on the synthesis and regulation of

advantageous ZnO nanocrystals which involved {001} facets, {101} facets and {100} facets on the same single crystal is still limited. In this article, we provided a simple, economical and environmentally friendly solvothermal method for fabrication of ZnO tetrakaidecahedrons exposed to varying percentages of {001}, {101}, and {100} facets using tetramethylammonium hydroxide (TMAH) as an addictive. The valence band (VB) and the conduction band (CB) positions of different crystal faces are


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different, so an interfacial heterojunction which can effectively enhance the photocatalytic activity is formed. The experimental results show that both the solvothermal reaction time and the amount of TMAH are selective for the formation of tetradecahedron. The increase in the solvothermal time results in an increase in crystal size while reducing the specific surface area. Large specific surface area can also provide more active sites to enhance photocatalytic activity. 2. Experimental method 2.1 Synthesis of ZnO tetrakaidecahedrons The reagents used in the experiment include zinc nitrate hexahydrate (Zn(NO3)2•6H2O), anhydrous ethanol and tetramethylammonium hydroxide (TMAH) without further purification. For the synthesis of ZnO-1 tetrakaidecahedrons, 0.01 mol of Zn(NO3)2•6H2O, 0.035 mol of TMAH and 60 mL absolute ethanol were taken in a 100 mL Teflon-lined stainless steel autoclave after become reaction mixture with constant stirring. The autoclave was sealed and maintained at 160 °C for 4 h. After being cooled, centrifuged and washed, the opalescent sample dried at 60 °C overnight. The amount of TMAH in the above process was changed to 0.02 mol, the reaction time was changed to 12 h, and the other processes remained unchanged to obtain ZnO-2 tetrakaidecahedrons. 2.2 Characterization The crystal phase of the sample was characterized using a Bruker D8 advanced X-ray powder diffractometer with Cu-Ka radiation (λ=1.5418 Å). The morphology and surface lattice characteristics of the samples were characterized by SEM, Hitachi S-4800 microscope and HRTEM, JEOL-2100, respectively. The Fourier transmission infrared (FTIR) spectra properties of the samples were characterized by FTIR spectrometer (IR Prestige-21, Shimadzu, Japan). The specific surface area of the samples was measured by Specific surface void analyzer (TriStar II


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3020 instrument). The photocurrent density of the sample was determined in 0.1 M Na2SO4 electrolyte by electrochemical system (CHI660E, Shanghai Chenhua Instrumental Co., Ltd. China). The obtained sample was used as a working electrode, platinum was used as a counter electrode, and Ag/AgCl was used as a reference electrode. 2.3 Measurement of photocatalysis Firstly, the MB solution with concentration of 20 mg/L was prepared for use. The 100 mL solution and 0.1 g ZnO tetrakaidecahedron were added to the beaker and treated in darkness for 1 h under continuous stirring. Then 2-3 mL of the solution was added to the test tube every 10 min under UV-visible light irradiation. The absorbance of the solution under different illumination time was measured by ultraviolet-visible spectrophotometry. 3. Results and discussion 3.1 Outline view of tetradecahedral ZnO particles The ZnO tetrakaidecahedrons was characterized by XRD, as shown in Figure 1. All of the diffraction peaks can be assigned to wurzite structure, in which lattice parameters a=3.250 Å and c=5.207 Å, space group P63mc (JCPDS No.36-1451). No diffraction peaks of other impurities were observed. The intensity ratio R(100)/(101) belonging to {100} and {101} planes for all the samples were depicted in Table 1. It can be seen from Figure 1 that the measured intensity ratio R(100)/(101) of ZnO-1 and ZnO-2 prepared in absolute ethanol with different content of TMAH were higher than the JCPDS No.36-1451 intensity ratio. And the ZnO-1 has a slightly larger intensity ratio than ZnO-2, as shown in Table 1. These results do indicate that the tetradecahedral ZnO nanoparticles are preferentially formed along a specific direction in the growth process.23-25


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Figure 1. XRD patterns of the synthesized ZnO tetrakaidecahedrons Table 1 Reaction recipe for the preparation of size-tunable ZnO tetrakaidecahedrons and their characterization data

Figure 2a and 2b present typical SEM images of the ZnO-1 products, which show that the products possess nice hexagonal tetrakaidecahedral morphology with a diameter of about 50–150 nm. The enlarged SEM image (Figure 2b inset) shows that the ZnO-1 tetrakaidecahedrons were enclosed by a majority of {101} and a minority of {100} facets. When growth condition of the crystals changed to initial solvothermal process, the shape of the crystal is altered, as shown in Figure 2c and 2d. Although, the crystals were still hexagonal phase tetradecahedral nanoparticles, their axial lengths are increased from 100 nm to 350 nm, and they show regular or the pencil stub-like morphologies. The size of the individual ZnO-2 particle is up to ~ 350 nm. Furthermore, the well- faceted particles can be easily identified as having {001}, {101} and


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{100} facets according to the SEM images, as well as the crystallographic symmetries of wurtzite phase.

Figure 2. Morphologies of size-tunable ZnO tetrakaidecahedrons by SEM: a-b) ZnO-1, c-d) ZnO-2; Inset: the corresponding front view of SEM images and simple model of individual particle

Figure 3. TEM images of tetradecahedral ZnO-1 NPs (TMAH=0.035 mol, 160 °C/ 4 h); (a) Low-resolution image, (b–c) high-resolution images showing staggered NPs, (d) Energy Dispersive X-Ray Spectroscopy (EDX) image, (e) Selected area electron diffraction spectrum (SAED) along [001]


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The performance of the crystal is closely related to the morphology because the morphology of polyhedral crystals is determined by the distribution of atoms in the crystal.26 The morphology of the polyhedron determines the exposure of the crystal plane and has an effect on photocatalysis. According to Euler's formula 27, the relationship between the vertex (V), the face (F) and the edge (E) of the synthesized tetradecahedral ZnO is V + F-E = 2. Therefore, the synthesized ZnO tetradecahedron has better geometric stability. For the tetradecahedral ZnO, there are three kinds of facets: six quadrangle facets, six isosceles trapezoid facets and two hexagon facets. As can be seen from the insets of Figures 2b and 2d, highly regular tetradecahedral ZnO nanoparticles are the only products. A tetrakaidecahedron has 14 faces, 18 vertices, and 30 edges, and its structure can be represented by the model in Figure 2. The samples were ground and ultrasonic treated in ethanol, and then the crystal surface of tetradecahedral ZnO-1 NPs was characterized by TEM. A typical TEM image analysis further showed that the product consisted of well dispersed hexagonal crystal nanostructures. As shown in Figures 3b and 3c, the lattice spacing of 0.28 nm, 0.26 nm, and 0.24 nm measured in the Figure correspond to the (100), (002), and (101) faces of ZnO-1 tetrakaidecahedrons, respectively. As shown in Figure 3d, the C, O, Zn and Cu elements was observed in the EDX plot, of which the Cu and C elements are derived from the sample preparation grid. Figure 3e shows the selected area electron diffraction along the [001] direction, from which it can be seen that ZnO belongs to a hexagonal crystal structure. TEM observation can reveal more details of ZnO-2 tetrakaidecahedron microstructure, as shown in Figure 4. Tetradecahedral ZnO-2 NPs is dark due to its large size (Figure 4a), indicating that the sample is composed of single crystal nanoparticles, and more evidence indicates that the nanoparticles are pseudo-regular tetrakaidecahedrons with pencil-like


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morphology. Figure 4b showed the HR-TEM image of the crystal edge. Figure 4c exhibited high resolution image of the ZnO-2 tetrakaidecahedrons measured at the position with black dashed box in Figure 4b. The lattice spacing of 0.28 nm corresponds to the d-spacing of the (100) crystal plane of ZnO-2 tetrakaidecahedron. Figure 5 showed the FT-IR analysis of different ZnO tetrakaidecahedrons in the range of 4000-400 cm-1 at room temperature. The broad absorption peak at 3200 ~ 3600 cm−1 observed in the two samples belongs to the O-H vibration peak of water. 28 The absorption at about 2347 cm-1 corresponds to C = O = C vibration, which is due to the presence of CO2 in the air. The relatively weak dissymmetric stretching mode of vibration of C-H was observed between 2781 and 3167 cm-1. The anti-symmetric stretching vibration and symmetric stretching vibration occurs between 1510 ~ 1210 cm−1 and 1060 ~ 1020 cm-1, indicating the vibration of NO3-. Moreover, the absorption peaks at 493 cm−1 (ZnO-1) and 476 cm−1 (ZnO-2) are attributed to the stretching mode of Zn-O.

Figure 4. TEM images (a) of ZnO-2 tetrakaidecahedrons and the high-resolution image (b, c) of the tetrakaidecahedron


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Figure 5. FTIR spectra of different ZnO nanostructures

Figure 6. SEM images of the samples taken out at different reaction stage. Each row corresponds to one sample: (a) and (b) 4h; (c) and (d) 8h; (e) and (f) 12h; (g) and (h) 24 h


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In addition, ZnO tetrakaidecahedron (0.02 TMAH) with different reaction times were analyzed by SEM. The corresponding SEM images were shown in Figure 6. The morphologies of the ZnO NPs at the reaction time of 4 h are all ZnO NPs hexagonal prisms with a diameter of about 200 nm, as shown in Figure 6a and 6b. A few bevel face architectures (dubbed {101} crystal face) of ZnO NPs can be observed when the reaction time reach to 8 h, as shown in Figure 6c and 6d. A large size of ZnO NPs was observed when the reaction time is 8 h and 12 h, as shown in Figure 6e. The morphology of ZnO varies greatly with the increase of solvothermal reaction time. The ratio of bevel crystal facets was greatly increased in the case of ZnO-2 NPs compared with the solvothermal reaction for 8 h. Pseudo-regular ZnO NPs were formed when the solvothermal reaction time was 24 h, as shown in Figure 6g. 3.2 Photocatalysis and photocurrent response

Figure 7. a and b are photocatalytic degradation diagrams for degrading MB and Rh B, respectively The photocatalytic activity of the samples is illustrated by the degradation of MB and Rh B dyes

under

UV-visible

light.

The

photocatalytic

performance

of

ZnO-1,

ZnO-2

tetrakaidecahedrons and the control group ( ZnO TMAH= 0.02 mol, 160 °C/4 h) was shown in Figure 7. Figures 7a and b showed the degradation rate of MB and Rh B in the presence and absence of catalysts. Where C is the concentration of the dye after irradiation time of t, and C0 is


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the concentration of dye after one hour of dark treatment. The self-degradation as the black line showed does not change obviously under a 300W Xe arc lamp irradiation for 30 min and 50 min, respectively. The MB degradation rate of the tetradecahedral ZnO-1 NPs can reach about 99 % in 30 min, while the control group and ZnO-2 tetrakaidecahedrons can reach 90 % and 83 % for the same time, respectively. At the same time, the Rh B degradation rate for the tetradecahedral ZnO-1 NPs can reach about 98 % in 50 min, while the control group and ZnO-2 tetrakaidecahedrons can approach about 82 % and 75 % for the same time, respectively. The experimental results revealed that ZnO-1 tetrakaidecahedrons still exhibited higher photocatalytic activities than ZnO-2 tetrakaidecahedrons in the process of dye degradation. The tetradecahedral ZnO simultaneously exposes the {001}, {101} and {100} crystal faces. It has been found that the valence band (VB) and the conduction band (CB) positions of different crystal faces are different, so that the interfacial heterojunction is generated to effectively suppress the recombination of electron-hole pairs. Bao et al. reported that the AgBr tetrakaidecahedrons with co-exposed {100} and {111} faces exhibited enhanced photocatalytic degradation due to the formation of facet heterojunctions. 29 It can be clearly seen from the SEM image that the exposed ratio of {001} surface of ZnO-1 and ZnO-2 tetrakaidecahedrons is reduced compared with the control group. The {101} plane exposure ratio of the ZnO-1 tetrakaidecahedrons is larger and the {100} plane is lower compared with the ZnO-2 tetrakaidecahedrons. The (001) plane has the highest energy in the crystal planes, the (101) plane is the second, and the (100) plane is the smallest. 30 Furthermore, the BET results showed that the specific surface areas of ZnO-1, ZnO-2 tetrakaidecahedrons and the control group are 17.71 m2/g, 4.76 m2/g and 11.58 m2/g, respectively. The large difference in activity is attributed to specific surface area and active surface exposure. The photocurrent density of the samples was shown in


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Figure 8. It can be seen that the photocurrent density of ZnO-1 tetrakaidecahedron is the highest, while that of ZnO-2 tetrakaidecahedron is the lowest, indicating that ZnO-1 tetrakaidecahedron has good photoelectric properties. It also can be seen from the Figure that no spike peak appears in all samples, indicating that the surface states cause less carrier recombination. Large specific surface area is conducive to providing more surfactant sites and scattering of incident light, thus improving photocatalytic activity. Therefore, these evidences show that the photocatalytic performance of ZnO tetrakaidecahedrons can be improved by adjusting the shape and specific surface area.

Figure 8. Transient photocurrent response of the samples

Figure 9. Schematic illustration for the formation of different tetradecahedral ZnO 3.3 Growth of ZnO tetrakaidecahedron nanoparticles


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The formation mechanism of ZnO tetrakaidecahedron by solvothermal method in the presence of zinc nitrate hexahydrate and TMAH is as follows: Zn(NO 3 )2  Zn 2   2NO 3 

N (CH 3 )4 OH





 (CH 3 )3 N  CH 3OH





NO 3  H 2O  2e -  NO 2  2OH Zn 2   4OH Zn(OH )4

2-



 Zn(OH )4

 ZnO  H 2O

2-



1 2 3 4  5

Nitrate anion (NO3-) and zinc cation (Zn2+) are produced in zinc nitrate absolute ethanol as shown in equation (1). Zn2+ is easy to react with OH- to form stable Zn(OH)42- complexes, just as equation (4). Zn(OH)42- complexes act as one of the growth units for the formation of ZnO NPs. Based on the above conclusions, the morphology of ZnO nanoparticles changes with solvothermal time and TMAH concentration as shown in Figure 9. From the graphs, the tetradecahedral ZnO particles are formed by adding six equivalent {101} planes on a tip of a hexagonal prism particle, that is, they are enclosed by six {101} planes, six {100} planes and two {001} planes. The growth of the nanostructures depends on the reaction time as well as the amount of TMAH. To some extent, many hexagonal prisms appear in the initial stage and further grow as the amount of TMAH increases. When the amount of TMAH is increased, the morphological change process is shown in the path (a), which becomes the NP of the tetradecahedral ZnO-1. The particle size of the sample decreases in path (a), and the exposure ratio of the {101} face increases significantly. The {100} face and {001} face are relatively reduced under the corrosion of TMAH. TMAH is an anisotropic etchant with excellent corrosion properties, good selectivity, non-toxic and environmentally friendly. The corrosion of the (100) planes of Si by TMAH has been reported in early stage.

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When the reaction time is increased,

the morphological change process is shown in the path (b), which becomes the NP of the tetradecahedral ZnO-2. In path (b), the {001} plane decreases and the {101} plane part increases,


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while the photocatalytic effect is lower than that of the control group, indicating that {001} plane and specific surface area have a greater influence on photocatalysis. Compared with ZnO-2, the exposure ratio of ZnO-1 {101} face is obviously increased, and the photocatalytic effect is also enhanced, indicating that the {101} face is more favorable for photocatalysis than {100} face. 4. Conclusion In summary, a wurtzite hexagonal ZnO tetrakaidecahedron exposing the {001}, {101} and {100} planes was prepared by using TMAH as an adjuvant. It was found that the amount of TMAH and the solvothermal reaction time have a great influence on the morphology of ZnO NPs. Combined with the photocatalytic results, we found that nanoparticles with large specific surface area and exposed more active facets({001}, {101} facets) are beneficial for photocatalysis. Acknowledgements This study was supported by the financial supports of the Key Research Project of Shandong Province (No. 2017GGX40121), the financial supports of the National Natural Science Foundation of China (No. 51402157, No. 51602164) and the Scientific Research Innovation Team in Colleges and Universities of Shandong Province. The authors declare no competing financial interest References (1) Han, X.; Jin, M.; Xie, S.; Kuang, Q.; Jiang, Z.; Jiang, Y.; Xie, Z.; Zheng, L. Synthesis of tin dioxide octahedral nanoparticles with exposed high-energy {221} facets and enhanced gassensing properties. Angew. Chem. Int. Ed. 2009, 48, 9180-3. (2) Zhao, Y.; Eley, C.; Hu, J.; Foord, J. S.; Ye, L.; He, H.; Tsang, S. C., Shape-dependent acidity and photocatalytic activity of Nb2O5 nanocrystals with an active TT (001) surface. Angew.


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For Table of Contents Use Only

ZnO tetrakaidecahedrons with co-exposed {001}, {101} and {100} facets: shape-selective synthesis and enhancing photocatalytic performance Yujie Liu, Dong Huang, Haixia Liu2*, Tianduo Li, Jingui Wang

Yujie Liu and Dong Huang are co-first authors of the article. * Corresponding author. E-mail address: [email protected] (H. Liu).


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The ZnO tetrakaidecahedron with different crystal faces ({001}, {101} and {100}) is efficiently synthesized by a simple solvothermal route with tetramethylammonium hydroxide (TMAH) as additive.


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ZnO tetrakaidecahedrons with co-exposed {001}, {101} and {100

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