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High photocatalytic property and crystal growth of spindle-like ZnO microparticles synthesized by one-step hydrothermal method
Accepted Manuscript High photocatalytic property and crystal growth of spindle-like ZnO microparticles synthesized by one-step hydrothermal method Xingjie Lv, Yi Du, Zhongfu Li, Zhongtao Chen, Kai Yang, Tong Liu, Chaofeng Zhu, Minxing Du, Yibing Feng PII:
S0042-207X(17)30436-0
DOI:
10.1016/j.vacuum.2017.08.007
Reference:
VAC 7532
To appear in:
Vacuum
Received Date: 7 April 2017 Revised Date:
4 August 2017
Accepted Date: 5 August 2017
Please cite this article as: Lv X, Du Y, Li Z, Chen Z, Yang K, Liu T, Zhu C, Du M, Feng Y, High photocatalytic property and crystal growth of spindle-like ZnO microparticles synthesized by one-step hydrothermal method, Vacuum (2017), doi: 10.1016/j.vacuum.2017.08.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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High photocatalytic property and crystal growth of spindle-like ZnO
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microparticles synthesized by one-step hydrothermal method
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Xingjie Lva, Yi Dua∗ Zhongfu Li a, Zhongtao Chen a, Kai Yang a, Tong Liu b, Chaofeng
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Zhua, Minxing Dua, Yibing Fenga
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a
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Ceramics of Shandong Province, Qilu University of Technology, Jinan 250353, P. R.
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China
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b
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R. China
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Abstract
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Key Laboratory of Processing and Testing Technology of Glass & Functional
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School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, P.
The spindle-like ZnO microparticles were successfully synthesized by a simple
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hydrothermal method using triethanolamine (TEA) as the surfactant. The phase
13
components, particle sizes and morphologies of ZnO samples derived from different
14
contents of TEA (3, 4, 5, 6 and 7 mL) were characterized by XRD, SEM, TEM, EDS
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and BET. The photocatalytic activity of ZnO microparticles with different
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morphologies was evaluated by degradation of methyl orange (MO). The
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photocatalytic degradation process was monitored in terms of decolorization and total
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organic carbon (TOC) removals. The results indicated that the spindle-like ZnO
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exhibited higher photodegradation efficiencies under UV light irradiation than other
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ZnO structures due to the special morphology. The photodegradation could be
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described as the pseudo-first-order kinetics with apparent rate constants ranging from
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∗
Corresponding author Tel.: +86 0531 89631233；Fax: +86 0531 89631226 E-mail address: [email protected] 1
ACCEPTED MANUSCRIPT 3.59 × 10−3 to 1.04 × 10−2 min−1, which were based on the morphology of the
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structures. The sample presented a good recycle performance.
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Keywords: Zinc oxide, Hydrothermal method, TEA, Spindle-like, Photocatalytic
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Introduction
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Recently, water pollution is one of the major threats to public health. Incorrect and
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excessive use of pesticides, fertilizers, and pigments has caused serious water
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pollution. The traditional methods for wastewater treatment, e.g., adsorption,
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membrane separation, and extraction, are inefficient and cost high energy. Therefore,
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the finding of a “green” technology for removal of pollutants from wastewater is a
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main challenging [1-3]. In the last decades, heterogeneous photocatalysts for
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application in wastewater treatment had been developed. Titanium dioxide (TiO2) is
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one of the most commonly used photocatalytic materials, which can be applied
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efficiently to degrade the organic pollutants in the wastewater [4]. However, the
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photoelectrons and photoholes of the TiO2 are inclined to combine during the
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photocatalytic process, thus the production rate of light quantum is less than 10%. So
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the photocatalytic efficiency of TiO2 is unsatisfactory, which restrict the practical
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applications of this photocatalyst. Zinc oxide (ZnO) possesses the similar band gap of
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TiO2, furthermore this semiconductor oxide is non-toxic and has high photocatalytic
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efficiency. So it can also be used as an effective candidate photocatalytic materials
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[5].
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Zinc oxide is a direct band-gap (Eg=3.37eV) semiconductor with good optics,
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electrical and structural characteristics [6]. ZnO nano-materials with different 2
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nanowhiskers [7-10], show many special properties, which has attracted much
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attention in recent years [11]. ZnO with different morphologies has been synthesized
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by various methods such as solvothermal method, template method, precipitation
5
method and green synthesis method [12-17] and has been used in many fields, for
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instance, solar cell, stealth material, luminescent material, sensor, photocatalyst and
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wastewater treatment[18-24]. The morphology of ZnO is affected by many factors, for
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example, reaction temperature, surfactant, zinc salt, solution concentration and the
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doping of semiconductors is demonstrated to be a promising method because of its
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high efficiency, convenience an briefness [25-28]. Among of them, surfactant is one of
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the most important factors affecting ZnO morphology. In previous studies,
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K.thingSsuriwong et al. [29] used the amino alcohols as the surfactant to synthesize
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ZnO microrods, which exhibited excellent band gap. The photocatalytic activity of
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ZnO is strongly related to the types of crystal, in the research of photocatalysis for
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methylene blue. Mclaren et al. [30] found that the catalytic performances of the polar
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surfaces (001) and (001) were higher than that of the non-polarized planes (010) and
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(210). However, the morphology of the microrods is irregular and uneven, so that
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those studies may be more reasonable if they had considered this situation. As we all
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know, photocatalytic activity could be enhanced by altering the shape and size
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distribution of ZnO, thus it is of great importance to find a straightforward method to
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prepare the samples with regular shape and uniform size.
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and
morphologies,
such
as
nanorods,
nanowries,
nanoflowers,
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Compared with the above synthesis method in the economic evaluation, we think 3
ACCEPTED MANUSCRIPT the synthesis of the spindle-like ZnO samples with hydrothermal method presented
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obvious advantages of uniform, stable, high yield and low cost. In this paper, a simple
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hydrothermal reaction with triethanolamine (TEA) as surfactant was used to
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synthesize spindle-like ZnO microstructures possessing superexcellent photocatalysis.
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The effect of the dosage of the TEA on the morphology and the photocatalytic
6
efficiency of the samples was investigated. Furthermore, the photocatalytic activities
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of the samples prepared with various amount of TEA were characterized with methyl
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orange (MO) as reacting substance. In addition, the formation mechanism of
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spindle-like ZnO was also discussed according to the evolution of morphologies from
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SEM.
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2.
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2.1. Preparation of ZnO samples
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Experimental
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The ZnO samples were synthesized by the traditional thermal hydrothermal method.
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All chemical reagents used were of analytical grade and purchased from Damao
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Corporation (Tianjin, China). Firstly, 5mmol Zn(CH3COO)2·2H2O was dissolved in a
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ethyl alcohol (15mL) and deionized water (45mL) mixed solution. Then 3, 4, 5, 6 or
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7mL surfactants triethanolamine (HOCH2CH2)3N, TEA) were dropwisely added into
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as-prepared Zn(CH3COO)2 solution under continuous stirring (denoted as sample A,
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B, C, D and E). After stirring for 30 min, 3 mL NH3·H2O was dropped into the
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obtained solution to adjust the pH value. At last, the resulted solution was transferred
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into a Teflon-lined stainless steel, reacted at 120°C for 8 h, and naturally cooled to
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room temperature. The produced white precipitant from hydrothermal reaction was
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for 3 cycles to remove other impurities. The resulted precipitants were dried at 60°C
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and then annealed at 500°C for 2h to form the expected ZnO microparticles.
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2.2. Characterization
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The morphology and element composition of microparticles were characterized by
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transmission electron microscope (TEM, JEOL, JEM-2100, Japan) and scanning
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electron microspore (SEM, Hitachi, S-4800, Japan) equipped with Energy-dispersive
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X-ray spectroscopy (EDS). The Brunauer–Emmett–Teller (BET) specific surface area
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of the products was analyzed by nitrogen adsorption in a Tristar3020 (Micromeritics
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Instrument Corp., USA) nitrogen adsorption apparatus. The phase composition of
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ZnO was determined using an X-ray diffractometer (XRD, Shimadzu, XRD-6100,
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Japan) using Cu-Kα radiation (λ = 0.1541nm) with a step scan of 0.1°/s from 20 to 70°
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at room temperature.
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2.3. Photocatalytic performance measurement
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Photocatalytic decomposition of methyl orange (MO) was used to evaluate the
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photocatalytic activities of the prepared ZnO microparticles. 30 mg ZnO catalyst was
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added to 50 mL MO solution (the concentration was 20mg/L) and ultrasonically
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dispersed for 10min. The obtained solution was stirred in a dark environment for
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30min to reach the adsorption-desorption equilibrium. The suspension containing MO
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and ZnO catalyst was irradiated by the UV light (365nm) in a XPA-7 (Xu Jiang,
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China) model photoreactor with continuous stirring to maintain uniform distribution
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of catalysts. The distance from the light source to the liquid level was 20cm.
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(Shimadzu, UV-2550, Japan). The TOC concentration was measured using TOC
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analyzer (TOC-LCPH, SHIMADZU, Japan). The MO concentration was analyzed by
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the UV absorbance. It is worth noting that the supernatant fluid was obtained at 10000
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rpm centrifugal for 10min. In addition, the photocatalytic degradation experiment was
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carried out with commercial zinc oxide (Haihua Co., Ltd, Shandong, China) as the
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reference.
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3. Results and discussion
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3.1. Photocatalytic activity
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Fig. 1 showed the photocatalytic activity of the nanostructured ZnO samples, which
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was analyzed by the degradation of MO solution under UV irradiation for 0, 30, 60,
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90, 120, 150 and 180 min. The “0 min” in the sample of this figure represents the MO
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dye stirred in a dark environment for 30 min to reach the adsorption desorption
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equilibrium. As shown in Fig. 1 the degradation rate was recorded by the different
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value of the absorption spectrum peaks at 463 nm and the degradation rate of ZnO
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with 5mL TEA was higher than the commercial ZnO and other samples. Degradation
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rate can be expressed as follows:
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ŋ =C × 100% C
3-1
0
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Where C0, C represent concentrations before and after UV light exposed [31]. The
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absorbance of MO solution was a linear relationship with the concentration of MO
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solution before 20 mg/L [32] as Fig. 2 showed. From the above picture, the sample C
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had the best photocatalytic performance. It was noteworthy that almost fully 6
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TEA (Fig. 3a). Therefore, 180 min has been chosen as the reaction period for
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discussion and comparison. Fig. 3b was the image of the solution under UV
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irradiation for different time. With increasing the photocatalytic reaction time, MO
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solution was gradually faded. When the UV irradiation sustain for 210 min, the MO
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solution was similar to water. As for sample C (with 5mL TEA), which presented a
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good recycle performance, photocatalytic efficiency still as high as 95% after 3 times
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cycles (Fig. 4).
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Fig. 5 showed the TOC removal efficiency (%) during the process of photocatalytic
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degradation. Mineralization rate can be expressed according to the value of TOC as
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follows:
Mineralization rate =
TOC0
× 100%
3-2
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Where TOC0 is the initial TOC concentration, TOCt is the TOC concentration at
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reaction time.
It was found that the total mineralization were 49.5, 51.2, 87.9, 48.3 and 45.5 after
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180 min by 3ml, 4ml, 5ml, 6ml and 7ml sample, respectively. The results indicated
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that color degradation was faster than the decrease of TOC. The low concentration of
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TOC in the solution indicated that the intermediate products of decolorization were
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resistant to photocatalytic degradation. Similar results were shown for other dyes.
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3.2 Kinetic analysis
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Using the normalized concentration vs. reaction time data, InTOC0/TOCt vs.
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reaction time plots were drawn and apparent rate constant (k) values were deduced 7
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from the slopes (Fig. 6). The corresponding kinetic date (kinetic equation, apparent
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reaction rate constant and correlation coefficient) for TOC reduction was presented in
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Tab.1. From the data in Fig. 6, InTOC0/TOCt vs. reaction time plots were found to be
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linear (R2 > 0.9).This indicates that TOC removal follows pseudo-first-order kinetics.
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3.3. UV-vis studies
The UV-vis absorption spectra of the as-prepared ZnO samples were shown in Fig.
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8. The spectra displayed a significant and obvious absorption of the light at
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wavelength less than 400 nm, and the absorption spectra were approximately similar.
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ZnO microrods sample showed a strong absorption in the ultraviolet light range, but
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no absorption can be found in the visible light range (wavelength: 400-800 nm). The
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data of UV-vis absorbance were transformed by apparatus software to diffuse
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reflectance, F(R), based on the Kubelka-Munk theory. Based on the relationship of
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incident photo energy with the absorption coefficient:
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(αhv) = B (hv -Eg)n
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3-2
where α is the absorption coefficient, hv is the energy of the incident photon, Eg is the
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band gap of the semiconductor material, as ZnO is a direct transition semiconductor,
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the value of n is 1/2. Since α is the proportional coefficient to F(R), that is to say:
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F(R) = α/s = (1-R)2/2R A = -lgR
3-3 3-4
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Where s is the scattering coefficient, A is the absorbance values. The energy intercept
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of the curve of [F(R)hv]2 vs hv gives Eg when the linear region is extrapolated to the 8
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zero ordinate, the band gap is calculated to be 3.20 eV as shown in the illustration
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(Fig. 8)[33-34].
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3.4. Powder X-ray diffraction study Fig. 9 showed the XRD patterns of the ZnO samples derived from different TEA
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addition. These samples showed the same pattern as XRD standard spectrum of ZnO
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(PDF#36-1451). The peaks located at 31.7°, 34.3° and 36.2° respectively correspond
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to (100), (002) and (101) lattice faces, which affirmed the hexagonal wurtzite
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structure of ZnO with lattice constants a =3.250 Å and c=5.207 Å. Generally, the
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crystallite size could be estimated according to the Scherrer’s equation [35]:
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D= Kλ/βcosθ
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3-5
where D = crystallite size, K = constant 0.89, λ = X-ray wavelength = 0.154, β = full
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width at half maximum intensity (FWHM) in radians and θ = diffraction angle in
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degrees. Then the size of samples were calculated to be 49, 34, 23, 40, 57nm.
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The sharpness of the peaks indicated that the products were well crystallized, and
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no impurity peaks were found in this XRD pattern indicating the high purity of the
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samples.
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3.5. SEM, TEM and EDS analysis
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The morphologies of the ZnO samples obtained with different addition of TEA
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were shown in Fig. 10. The sample with 3 mL TEA has incomplete morphology and
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the crystal growth was not completed as seen in Fig. 10a. With the amount of TEA
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increased to 4 mL, the assembled ZnO with irregular shape was transformed into
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hexagonal structure and (001) facet was exposed out (Fig. 10b). When the surfactant 9
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ZnO sample was achieved, and spindle-like ZnO was obtained under this condition
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(Fig. 10c). The crystal surface with excellent photocatalytic performance was shown
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to the maximum extent. (Fig. 10f) and the size of the crystallite on the surface was
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about 25 nm, the value was similar to the above XRD results. When the dosage of
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TEA reaches 6mL, the {100} and {110} planes grow excessively and the morphology
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became irregular (Fig. 10d). When the addition amount of TEA is 7 mL, the shapes of
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some ZnO samples become spheroidic shape (Fig. 10e). In conclusion, with the
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increase of the amount of tea, the adsorption of TEA on the surface of surface
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increased, the growth rate of crystal face increases at the same time, and the shape
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changed from irregular to spindle shape and then to spheroidic shape. The TEM
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images of these samples showed that the contour of zinc oxide was the same as that in
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SEM images, and the outer part of the individual microparticles was the same as the
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inner part along the grains, which suggested that the inner part of the grains was not
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hollow.
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As shown in Fig .11, the corresponding high-resolution transmission electron
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microscopy (HRTEM) micrograph of simple C indicated that the nanosheets consisted
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of nanoparticles with the different crystallographic orientation. The periodic fringe
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spacing of 2.6 Å and 2.4 Å corresponded to the d-spacing of (002) and (101) planes.
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The continuous and clear diffraction rings at surface such as (002), (102) and (101) of
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SAED results (the inset of Fig. 11) showed that ZnO nanoparticles were
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polycrystalline structure.
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analyzing the test results, a table (Tab. 2) that contained the contents of each
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component element was obtained. It can be seen from the table that the sample only
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has two elements Zn and O, and the atomic percentage of them were almost same,
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proving that the sample was Zinc Oxide, which was further confirmed by the XRD
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analysis (Fig. 9).
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The relevant data of the morphology, particle size, crystallite size and photocatalytic
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activity of the five catalyst systems have been summarized in Tab. 3. The sample C
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had the smallest crystallite size and the best photocatalytic activity.
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3.6 BET analysis
The surface area and pore volume of the photocatalyst were investigated using
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nitrogen gas adsorption–desorption method. To characterize the specific surface area
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and porosity, nitrogen sorption measurements of the samples were carried out at 77 K.
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Nitrogen adsorption–desorption isothermal and the corresponding Barrett–Joyner–
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Halenda (BJH) pore size distribution desorption were presented in Figure 13. The
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pore volume distribution curve of spindle-like ZnO (a) and commercial ZnO (b) was
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given as an inset image in Figure 13. The profile can be categorized to type III
18
isotherm.
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Table 4 showed the results of the measured BET specific surface area and BJH pore size distribution desorption of the spindle-like ZnO and commercial ZnO.
As can be seen in Table 4, spindle-like ZnO had a largest specific surface area of 11
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higher than commercial ZnO.
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3.6. Crystal growth
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It has been found that the growth rate of the (hkl) surface (Rhkl) was determined by
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surface area (Ahkl) and total bonding energy (Ethkl, include the crystal surface binding
6
energy (EShkl) and the phase bond energy (Ebhkl)). The surface bonding energy density (e
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S hkl
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The smaller the bulk bonding energy, the greater the activity of the growth unit, the
9
more conducive to the growth unit and the surface of the effective growth of the role
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of bonding. Hence, the relative growth rate of the crystal surface under the control of
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the surface bond can be expressed as:
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= EShkl / Ahkl) mean the (hkl) crystal surface's ability to attract bonding growth unit.
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The relationship between surface bond energy density and surface bonding energy as:
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∝
3-7
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And the relationship of the total bonding energy, surface bonding energy and phase
16
bond energy:
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18
= −
Therefore, the surface growth rate can be expressed as:
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3-8
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∝
1
3-9
( )
It can be seen from the formula that the most important factor influencing the
3
crystal surface growth rate is crystal growth surface bonding energy in the growth
4
process. After analysis of the ZnO hexagonal crystal structure, the low index crystal
5
faces {100}, {110}, {011}, (001) and (001) were determined to be the calculating
6
factors. After analysis of each crystal plane property of chemical bond, the surface
7
growth rates of the above five crystal faces are shown in the Tab. 5 [36]. Based on the
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size of these low index surfaces growth rate, R(001) ＞ R{110} ＞ R{011} ＞ R{100} ＞
9
R(001)，a hexagonal pencil-like ZnO thermodynamics ideal form could be obtained by
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simulating (Fig. 14a). The hexagonal pencil-like ZnO ideal forms were surrounded by
11
{100}, {011} and (001) crystal faces. When the R(001) gets lower, the (001) crystal
12
face was exposed, the shape of ZnO crystal shows like truncated dodecagon pencil
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(Fig. 14b). When the R{110} gets lower, the {110} crystal face was exposed, the shape
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of ZnO crystal shows like dodecagon pencil (Fig. 14c)[37].
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In this hydrothermal reaction, the TEA has two different effects. On one hand, TEA
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as a kind of surface active agent can affect the growth of different directions[38], on
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the other hand, TEA can slowly release OH- by hydrolysis and changed the pH value
18
of the mixed solution [39]. During the hydrothermal process, TEA acting as a kind of
19
cationic surfactant can be complex with growth units [Zn(OH)4]2-, further combined
20
with ZnO crystal nucleus to form stable and tiny ZnO microparticles.[40] In order to
21
make the structure stable, these ZnO microparticles tend to be gathered to form
22
hexagonal-pyramid
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area
and
surface
energy.
The
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polarity surface (000 1 ) of ZnO pyamid by coulomb interaction. And the
3
N(CH2CH2OH)3-H+ on the (0001) surface as a buffer layer further absorbed growth
4
units. Then another relatively small pyramid grew on the (0001) surface, with the
5
increase of the time of hydrothermal reaction, the small pyramid grew up, then the
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two pieces of the ZnO nanocrystalline became the same, finally a spindle-like ZnO
7
microstructure was formed.
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Conclusion
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In this research, one-step hydrothermal method was introduced to synthesize
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different morphologies of ZnO structures. Compared to microsphere structures,
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microrod structures, commercial structures and spindle-like structures exhibited
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outstanding photocatalytic activities due to their unique structures. It was found that
13
the photodegradation of MO followed the pseudo-first-order kinetics. A model was
14
developed to prove that the concentration of TEA played a significant impetus on the
15
growth and forming of the （001） and （001）crystals of ZnO microrods to form the
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spindle-like structures.
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Acknowledgements
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This work is financially supported by the Science and Technology Development
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Plan Project of Shandong Province, China (Grant No.2013GSF11714). The authors
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declare that they have no conflict of interest.
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oxide based photocatalyst in water treatment technology: A review. Water Res 88
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as organic corrosion inhibitor into nanoparticles and its active corrosion
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protection for steel sheets, Surf. Coat. Tech. 206 (2012) 2354–2362. [39] K. Biswas, B. Das, C. N. R. Rao, Growth kinetics of ZnO nanorods:
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[40] Merianos J J. Surface-active agents. Disinfection, sterilization, and preservation.
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Figure captions
2
Fig.1 The different photocatalytic degradation curves of MO, without any ZnO
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sample, with commercial ZnO and with the synthetic samples.
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Fig.2 Linear relationship of MO solution concentration and absorbance (R2
6
=0.99986).
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Fig.3 The absorbance of MO solution under UV irradiation for different time (a), and
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the corresponding photos of the solution (b).
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Fig.4 Recycling experiments of visible-light photocatalytic degradation of MO over
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the spindle-like ZnO microparticles (light source: 500 W of tungsten lamp; catalyst
13
dose: 30 mg; reaction time: 180 min; [MO]: 20 mg/ L; volume 50 mL).
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Fig.5 Relationship between degradation of methyl orange and degradation time: TOC
16
removal efficiency.
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Fig.6 Relationship between ln TOC0/TOCt and irradiation time of different ZnO
19
structures.
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Fig.7 UV–vis absorption spectra of as-prepared ZnO samples.
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Fig.8 UV–vis absorption spectra of sample C and the tangent curve of the forbidden
2
band width.
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Fig.9 XRD patterns of the synthesized ZnO (a) with 3mL TEA (b) with 4mL TEA (c)
5
with 5mL TEA (d) with 6 mL TEA and (e) with 7mL TEA.
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Fig.10 SEM and TEM images of ZnO prepared (a) with 3mL TEA (b) with 4mL TEA
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(c) with 5mL TEA (d) with 6mL TEA and (e) with 7mL TEA, (f) one crystal grain of
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sample C.
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Fig.11 HRTEM images of ZnO with 5mL TEA and Selected-area electron diffraction
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patterns of the same samples.
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Fig.12 EDS image of the 5mL TEA addition ZnO sample.
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Fig.13 N2 adsorption–desorption isotherms and pore size distribution curves of
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spindle-like ZnO (a) and commercial ZnO (b).
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Fig.14 (a) The pencil-like morphology as the ideal morphology of ZnO crystal, (b)
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(001) surface appears when the surface growth rate along <001> direction is low, (c)
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{110} surface appears when the surface growth rate along <110> direction is low.
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Table legends
2
Table 1 Kinetic parameters for TOC reduction of MO by different morphological ZnO
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structures.
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Table 2 The EDS results of the samples.
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Table 3 Morphology, particle size, crystallite size and catalytic activity of the three
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ZnO samples.
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Table 4 Surface properties of spindle-like ZnO and commercial ZnO.
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Table 5 The surface growth rates of {100}, {110}, {011}, (001) and (001) of ZnO
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crystal.
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R2
Kinetic equation
A (3ml)
In (TOC0/TOCt) = 0.00388t – 0.00934
0.00388
0.99722
B (4ml)
In (TOC0/TOCt) = 0.00417t – 0.00624
0.00417
0.99438
C (5ml)
In (TOC0/TOCt) = 0.01042t – 0.01735
0.01042
0.99532
D (6ml)
In (TOC0/TOCt) = 0.00384t – 0.01604
0.00384
0.99336
E( 7ml)
In (TOC0/TOCt) = 0.00359t – 0.02471
0.00359
0.98861
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ZnO samples
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percentage by weight
atomic percentage
C
0.34
1.15
19.72
49.62
79.93
49.23
100
100
O Zn
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Table 3 Morphology, particle size, crystallite size and catalytic activity of the three ZnO samples. Particle size (µm)
Crystallite size (nm)
MO solution degradation rate in 180 min (%)
49
62.40
3-5
34
68.47
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Morphology
Irregular polyhedron Less microrods， microspheres and Irregular polyhedron
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ZnO sample
2-4
C
Spindle-like
5-8
23
97.67
D
Microspheres and less microrods
3-6
40
65.32
E
Microspheres
4-6
57
55.79
A
B
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Table 4 Surface properties of spindle-like ZnO and commercial ZnO. Total pore volume (cm3/g)
7.93
0.086
spindle-like ZnO commercial ZnO
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0.003
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crystal.
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Superficial area Ahkl(Å2)
Surface bonding energy S Ehkl（kcal/mol）
Total bonding energy t Ehkl（kcal/mol）
{100}
33.845
83.945
663.221
Relative growth rate of surface Rhkl 1
{110}
55.052
167.891
663.221
1.44
{011}
38.465
125.918
663.221
1.42
(001)
18.291
125.918
663.221
2.99
39.887
663.221
0.82
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Highlights The morphology of ZnO microparticles was spindle-like. A simple one-step hydrothermal method was used to synthesize the specimen.
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The product presented a high photocatalytic performance and excellent recycle performance.
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The growth mechanism of the spindle-like ZnO microparticles was discussed.
S0042-207X(17)30436-0
DOI:
10.1016/j.vacuum.2017.08.007
Reference:
VAC 7532
To appear in:
Vacuum
Received Date: 7 April 2017 Revised Date:
4 August 2017
Accepted Date: 5 August 2017
Please cite this article as: Lv X, Du Y, Li Z, Chen Z, Yang K, Liu T, Zhu C, Du M, Feng Y, High photocatalytic property and crystal growth of spindle-like ZnO microparticles synthesized by one-step hydrothermal method, Vacuum (2017), doi: 10.1016/j.vacuum.2017.08.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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High photocatalytic property and crystal growth of spindle-like ZnO
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microparticles synthesized by one-step hydrothermal method
3
Xingjie Lva, Yi Dua∗ Zhongfu Li a, Zhongtao Chen a, Kai Yang a, Tong Liu b, Chaofeng
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Zhua, Minxing Dua, Yibing Fenga
5
a
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Ceramics of Shandong Province, Qilu University of Technology, Jinan 250353, P. R.
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China
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b
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R. China
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Abstract
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Key Laboratory of Processing and Testing Technology of Glass & Functional
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School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, P.
The spindle-like ZnO microparticles were successfully synthesized by a simple
12
hydrothermal method using triethanolamine (TEA) as the surfactant. The phase
13
components, particle sizes and morphologies of ZnO samples derived from different
14
contents of TEA (3, 4, 5, 6 and 7 mL) were characterized by XRD, SEM, TEM, EDS
15
and BET. The photocatalytic activity of ZnO microparticles with different
16
morphologies was evaluated by degradation of methyl orange (MO). The
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photocatalytic degradation process was monitored in terms of decolorization and total
18
organic carbon (TOC) removals. The results indicated that the spindle-like ZnO
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exhibited higher photodegradation efficiencies under UV light irradiation than other
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ZnO structures due to the special morphology. The photodegradation could be
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described as the pseudo-first-order kinetics with apparent rate constants ranging from
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∗
Corresponding author Tel.: +86 0531 89631233；Fax: +86 0531 89631226 E-mail address: [email protected] 1
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2
structures. The sample presented a good recycle performance.
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Keywords: Zinc oxide, Hydrothermal method, TEA, Spindle-like, Photocatalytic
4
Introduction
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Recently, water pollution is one of the major threats to public health. Incorrect and
6
excessive use of pesticides, fertilizers, and pigments has caused serious water
7
pollution. The traditional methods for wastewater treatment, e.g., adsorption,
8
membrane separation, and extraction, are inefficient and cost high energy. Therefore,
9
the finding of a “green” technology for removal of pollutants from wastewater is a
10
main challenging [1-3]. In the last decades, heterogeneous photocatalysts for
11
application in wastewater treatment had been developed. Titanium dioxide (TiO2) is
12
one of the most commonly used photocatalytic materials, which can be applied
13
efficiently to degrade the organic pollutants in the wastewater [4]. However, the
14
photoelectrons and photoholes of the TiO2 are inclined to combine during the
15
photocatalytic process, thus the production rate of light quantum is less than 10%. So
16
the photocatalytic efficiency of TiO2 is unsatisfactory, which restrict the practical
17
applications of this photocatalyst. Zinc oxide (ZnO) possesses the similar band gap of
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TiO2, furthermore this semiconductor oxide is non-toxic and has high photocatalytic
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efficiency. So it can also be used as an effective candidate photocatalytic materials
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[5].
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Zinc oxide is a direct band-gap (Eg=3.37eV) semiconductor with good optics,
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electrical and structural characteristics [6]. ZnO nano-materials with different 2
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nanowhiskers [7-10], show many special properties, which has attracted much
3
attention in recent years [11]. ZnO with different morphologies has been synthesized
4
by various methods such as solvothermal method, template method, precipitation
5
method and green synthesis method [12-17] and has been used in many fields, for
6
instance, solar cell, stealth material, luminescent material, sensor, photocatalyst and
7
wastewater treatment[18-24]. The morphology of ZnO is affected by many factors, for
8
example, reaction temperature, surfactant, zinc salt, solution concentration and the
9
doping of semiconductors is demonstrated to be a promising method because of its
10
high efficiency, convenience an briefness [25-28]. Among of them, surfactant is one of
11
the most important factors affecting ZnO morphology. In previous studies,
12
K.thingSsuriwong et al. [29] used the amino alcohols as the surfactant to synthesize
13
ZnO microrods, which exhibited excellent band gap. The photocatalytic activity of
14
ZnO is strongly related to the types of crystal, in the research of photocatalysis for
15
methylene blue. Mclaren et al. [30] found that the catalytic performances of the polar
16
surfaces (001) and (001) were higher than that of the non-polarized planes (010) and
17
(210). However, the morphology of the microrods is irregular and uneven, so that
18
those studies may be more reasonable if they had considered this situation. As we all
19
know, photocatalytic activity could be enhanced by altering the shape and size
20
distribution of ZnO, thus it is of great importance to find a straightforward method to
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prepare the samples with regular shape and uniform size.
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and
morphologies,
such
as
nanorods,
nanowries,
nanoflowers,
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Compared with the above synthesis method in the economic evaluation, we think 3
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2
obvious advantages of uniform, stable, high yield and low cost. In this paper, a simple
3
hydrothermal reaction with triethanolamine (TEA) as surfactant was used to
4
synthesize spindle-like ZnO microstructures possessing superexcellent photocatalysis.
5
The effect of the dosage of the TEA on the morphology and the photocatalytic
6
efficiency of the samples was investigated. Furthermore, the photocatalytic activities
7
of the samples prepared with various amount of TEA were characterized with methyl
8
orange (MO) as reacting substance. In addition, the formation mechanism of
9
spindle-like ZnO was also discussed according to the evolution of morphologies from
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SEM.
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2.
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2.1. Preparation of ZnO samples
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The ZnO samples were synthesized by the traditional thermal hydrothermal method.
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All chemical reagents used were of analytical grade and purchased from Damao
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Corporation (Tianjin, China). Firstly, 5mmol Zn(CH3COO)2·2H2O was dissolved in a
16
ethyl alcohol (15mL) and deionized water (45mL) mixed solution. Then 3, 4, 5, 6 or
17
7mL surfactants triethanolamine (HOCH2CH2)3N, TEA) were dropwisely added into
18
as-prepared Zn(CH3COO)2 solution under continuous stirring (denoted as sample A,
19
B, C, D and E). After stirring for 30 min, 3 mL NH3·H2O was dropped into the
20
obtained solution to adjust the pH value. At last, the resulted solution was transferred
21
into a Teflon-lined stainless steel, reacted at 120°C for 8 h, and naturally cooled to
22
room temperature. The produced white precipitant from hydrothermal reaction was
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for 3 cycles to remove other impurities. The resulted precipitants were dried at 60°C
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and then annealed at 500°C for 2h to form the expected ZnO microparticles.
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2.2. Characterization
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The morphology and element composition of microparticles were characterized by
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transmission electron microscope (TEM, JEOL, JEM-2100, Japan) and scanning
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electron microspore (SEM, Hitachi, S-4800, Japan) equipped with Energy-dispersive
8
X-ray spectroscopy (EDS). The Brunauer–Emmett–Teller (BET) specific surface area
9
of the products was analyzed by nitrogen adsorption in a Tristar3020 (Micromeritics
10
Instrument Corp., USA) nitrogen adsorption apparatus. The phase composition of
11
ZnO was determined using an X-ray diffractometer (XRD, Shimadzu, XRD-6100,
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Japan) using Cu-Kα radiation (λ = 0.1541nm) with a step scan of 0.1°/s from 20 to 70°
13
at room temperature.
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2.3. Photocatalytic performance measurement
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Photocatalytic decomposition of methyl orange (MO) was used to evaluate the
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photocatalytic activities of the prepared ZnO microparticles. 30 mg ZnO catalyst was
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added to 50 mL MO solution (the concentration was 20mg/L) and ultrasonically
18
dispersed for 10min. The obtained solution was stirred in a dark environment for
19
30min to reach the adsorption-desorption equilibrium. The suspension containing MO
20
and ZnO catalyst was irradiated by the UV light (365nm) in a XPA-7 (Xu Jiang,
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China) model photoreactor with continuous stirring to maintain uniform distribution
22
of catalysts. The distance from the light source to the liquid level was 20cm.
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(Shimadzu, UV-2550, Japan). The TOC concentration was measured using TOC
3
analyzer (TOC-LCPH, SHIMADZU, Japan). The MO concentration was analyzed by
4
the UV absorbance. It is worth noting that the supernatant fluid was obtained at 10000
5
rpm centrifugal for 10min. In addition, the photocatalytic degradation experiment was
6
carried out with commercial zinc oxide (Haihua Co., Ltd, Shandong, China) as the
7
reference.
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3. Results and discussion
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3.1. Photocatalytic activity
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Fig. 1 showed the photocatalytic activity of the nanostructured ZnO samples, which
11
was analyzed by the degradation of MO solution under UV irradiation for 0, 30, 60,
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90, 120, 150 and 180 min. The “0 min” in the sample of this figure represents the MO
13
dye stirred in a dark environment for 30 min to reach the adsorption desorption
14
equilibrium. As shown in Fig. 1 the degradation rate was recorded by the different
15
value of the absorption spectrum peaks at 463 nm and the degradation rate of ZnO
16
with 5mL TEA was higher than the commercial ZnO and other samples. Degradation
17
rate can be expressed as follows:
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ŋ =C × 100% C
3-1
0
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Where C0, C represent concentrations before and after UV light exposed [31]. The
20
absorbance of MO solution was a linear relationship with the concentration of MO
21
solution before 20 mg/L [32] as Fig. 2 showed. From the above picture, the sample C
22
had the best photocatalytic performance. It was noteworthy that almost fully 6
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2
TEA (Fig. 3a). Therefore, 180 min has been chosen as the reaction period for
3
discussion and comparison. Fig. 3b was the image of the solution under UV
4
irradiation for different time. With increasing the photocatalytic reaction time, MO
5
solution was gradually faded. When the UV irradiation sustain for 210 min, the MO
6
solution was similar to water. As for sample C (with 5mL TEA), which presented a
7
good recycle performance, photocatalytic efficiency still as high as 95% after 3 times
8
cycles (Fig. 4).
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Fig. 5 showed the TOC removal efficiency (%) during the process of photocatalytic
10
degradation. Mineralization rate can be expressed according to the value of TOC as
11
follows:
Mineralization rate =
TOC0
× 100%
3-2
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12 13
Where TOC0 is the initial TOC concentration, TOCt is the TOC concentration at
14
reaction time.
It was found that the total mineralization were 49.5, 51.2, 87.9, 48.3 and 45.5 after
16
180 min by 3ml, 4ml, 5ml, 6ml and 7ml sample, respectively. The results indicated
17
that color degradation was faster than the decrease of TOC. The low concentration of
18
TOC in the solution indicated that the intermediate products of decolorization were
19
resistant to photocatalytic degradation. Similar results were shown for other dyes.
20
3.2 Kinetic analysis
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21
Using the normalized concentration vs. reaction time data, InTOC0/TOCt vs.
22
reaction time plots were drawn and apparent rate constant (k) values were deduced 7
ACCEPTED MANUSCRIPT 1
from the slopes (Fig. 6). The corresponding kinetic date (kinetic equation, apparent
2
reaction rate constant and correlation coefficient) for TOC reduction was presented in
3
Tab.1. From the data in Fig. 6, InTOC0/TOCt vs. reaction time plots were found to be
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linear (R2 > 0.9).This indicates that TOC removal follows pseudo-first-order kinetics.
6
3.3. UV-vis studies
The UV-vis absorption spectra of the as-prepared ZnO samples were shown in Fig.
8
8. The spectra displayed a significant and obvious absorption of the light at
9
wavelength less than 400 nm, and the absorption spectra were approximately similar.
10
ZnO microrods sample showed a strong absorption in the ultraviolet light range, but
11
no absorption can be found in the visible light range (wavelength: 400-800 nm). The
12
data of UV-vis absorbance were transformed by apparatus software to diffuse
13
reflectance, F(R), based on the Kubelka-Munk theory. Based on the relationship of
14
incident photo energy with the absorption coefficient:
15
(αhv) = B (hv -Eg)n
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7
3-2
where α is the absorption coefficient, hv is the energy of the incident photon, Eg is the
17
band gap of the semiconductor material, as ZnO is a direct transition semiconductor,
18
the value of n is 1/2. Since α is the proportional coefficient to F(R), that is to say:
19 20
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F(R) = α/s = (1-R)2/2R A = -lgR
3-3 3-4
21
Where s is the scattering coefficient, A is the absorbance values. The energy intercept
22
of the curve of [F(R)hv]2 vs hv gives Eg when the linear region is extrapolated to the 8
ACCEPTED MANUSCRIPT 1
zero ordinate, the band gap is calculated to be 3.20 eV as shown in the illustration
2
(Fig. 8)[33-34].
3
3.4. Powder X-ray diffraction study Fig. 9 showed the XRD patterns of the ZnO samples derived from different TEA
5
addition. These samples showed the same pattern as XRD standard spectrum of ZnO
6
(PDF#36-1451). The peaks located at 31.7°, 34.3° and 36.2° respectively correspond
7
to (100), (002) and (101) lattice faces, which affirmed the hexagonal wurtzite
8
structure of ZnO with lattice constants a =3.250 Å and c=5.207 Å. Generally, the
9
crystallite size could be estimated according to the Scherrer’s equation [35]:
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D= Kλ/βcosθ
10
3-5
where D = crystallite size, K = constant 0.89, λ = X-ray wavelength = 0.154, β = full
12
width at half maximum intensity (FWHM) in radians and θ = diffraction angle in
13
degrees. Then the size of samples were calculated to be 49, 34, 23, 40, 57nm.
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The sharpness of the peaks indicated that the products were well crystallized, and
15
no impurity peaks were found in this XRD pattern indicating the high purity of the
16
samples.
17
3.5. SEM, TEM and EDS analysis
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The morphologies of the ZnO samples obtained with different addition of TEA
19
were shown in Fig. 10. The sample with 3 mL TEA has incomplete morphology and
20
the crystal growth was not completed as seen in Fig. 10a. With the amount of TEA
21
increased to 4 mL, the assembled ZnO with irregular shape was transformed into
22
hexagonal structure and (001) facet was exposed out (Fig. 10b). When the surfactant 9
ACCEPTED MANUSCRIPT TEA was increased to 5 mL, the optimal synthesis condition for the self-assemble
2
ZnO sample was achieved, and spindle-like ZnO was obtained under this condition
3
(Fig. 10c). The crystal surface with excellent photocatalytic performance was shown
4
to the maximum extent. (Fig. 10f) and the size of the crystallite on the surface was
5
about 25 nm, the value was similar to the above XRD results. When the dosage of
6
TEA reaches 6mL, the {100} and {110} planes grow excessively and the morphology
7
became irregular (Fig. 10d). When the addition amount of TEA is 7 mL, the shapes of
8
some ZnO samples become spheroidic shape (Fig. 10e). In conclusion, with the
9
increase of the amount of tea, the adsorption of TEA on the surface of surface
10
increased, the growth rate of crystal face increases at the same time, and the shape
11
changed from irregular to spindle shape and then to spheroidic shape. The TEM
12
images of these samples showed that the contour of zinc oxide was the same as that in
13
SEM images, and the outer part of the individual microparticles was the same as the
14
inner part along the grains, which suggested that the inner part of the grains was not
15
hollow.
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As shown in Fig .11, the corresponding high-resolution transmission electron
17
microscopy (HRTEM) micrograph of simple C indicated that the nanosheets consisted
18
of nanoparticles with the different crystallographic orientation. The periodic fringe
19
spacing of 2.6 Å and 2.4 Å corresponded to the d-spacing of (002) and (101) planes.
20
The continuous and clear diffraction rings at surface such as (002), (102) and (101) of
21
SAED results (the inset of Fig. 11) showed that ZnO nanoparticles were
22
polycrystalline structure.
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ACCEPTED MANUSCRIPT Fig. 12 showed the EDS image of the sample with 5mL TEA as surfactant. By
2
analyzing the test results, a table (Tab. 2) that contained the contents of each
3
component element was obtained. It can be seen from the table that the sample only
4
has two elements Zn and O, and the atomic percentage of them were almost same,
5
proving that the sample was Zinc Oxide, which was further confirmed by the XRD
6
analysis (Fig. 9).
7
The relevant data of the morphology, particle size, crystallite size and photocatalytic
8
activity of the five catalyst systems have been summarized in Tab. 3. The sample C
9
had the smallest crystallite size and the best photocatalytic activity.
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3.6 BET analysis
The surface area and pore volume of the photocatalyst were investigated using
12
nitrogen gas adsorption–desorption method. To characterize the specific surface area
13
and porosity, nitrogen sorption measurements of the samples were carried out at 77 K.
14
Nitrogen adsorption–desorption isothermal and the corresponding Barrett–Joyner–
15
Halenda (BJH) pore size distribution desorption were presented in Figure 13. The
16
pore volume distribution curve of spindle-like ZnO (a) and commercial ZnO (b) was
17
given as an inset image in Figure 13. The profile can be categorized to type III
18
isotherm.
19 20
21
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Table 4 showed the results of the measured BET specific surface area and BJH pore size distribution desorption of the spindle-like ZnO and commercial ZnO.
As can be seen in Table 4, spindle-like ZnO had a largest specific surface area of 11
ACCEPTED MANUSCRIPT 7.93 m2/g. The surface area and pore volume of spindle-shaped ZnO were much
2
higher than commercial ZnO.
3
3.6. Crystal growth
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It has been found that the growth rate of the (hkl) surface (Rhkl) was determined by
5
surface area (Ahkl) and total bonding energy (Ethkl, include the crystal surface binding
6
energy (EShkl) and the phase bond energy (Ebhkl)). The surface bonding energy density (e
7
S hkl
8
The smaller the bulk bonding energy, the greater the activity of the growth unit, the
9
more conducive to the growth unit and the surface of the effective growth of the role
10
of bonding. Hence, the relative growth rate of the crystal surface under the control of
11
the surface bond can be expressed as:
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4
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= EShkl / Ahkl) mean the (hkl) crystal surface's ability to attract bonding growth unit.
3-6
The relationship between surface bond energy density and surface bonding energy as:
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∝
12
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14
∝
3-7
15
And the relationship of the total bonding energy, surface bonding energy and phase
16
bond energy:
17
18
= −
Therefore, the surface growth rate can be expressed as:
12
3-8
ACCEPTED MANUSCRIPT
∝
1
3-9
( )
It can be seen from the formula that the most important factor influencing the
3
crystal surface growth rate is crystal growth surface bonding energy in the growth
4
process. After analysis of the ZnO hexagonal crystal structure, the low index crystal
5
faces {100}, {110}, {011}, (001) and (001) were determined to be the calculating
6
factors. After analysis of each crystal plane property of chemical bond, the surface
7
growth rates of the above five crystal faces are shown in the Tab. 5 [36]. Based on the
8
size of these low index surfaces growth rate, R(001) ＞ R{110} ＞ R{011} ＞ R{100} ＞
9
R(001)，a hexagonal pencil-like ZnO thermodynamics ideal form could be obtained by
10
simulating (Fig. 14a). The hexagonal pencil-like ZnO ideal forms were surrounded by
11
{100}, {011} and (001) crystal faces. When the R(001) gets lower, the (001) crystal
12
face was exposed, the shape of ZnO crystal shows like truncated dodecagon pencil
13
(Fig. 14b). When the R{110} gets lower, the {110} crystal face was exposed, the shape
14
of ZnO crystal shows like dodecagon pencil (Fig. 14c)[37].
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2
In this hydrothermal reaction, the TEA has two different effects. On one hand, TEA
16
as a kind of surface active agent can affect the growth of different directions[38], on
17
the other hand, TEA can slowly release OH- by hydrolysis and changed the pH value
18
of the mixed solution [39]. During the hydrothermal process, TEA acting as a kind of
19
cationic surfactant can be complex with growth units [Zn(OH)4]2-, further combined
20
with ZnO crystal nucleus to form stable and tiny ZnO microparticles.[40] In order to
21
make the structure stable, these ZnO microparticles tend to be gathered to form
22
hexagonal-pyramid
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15
to
reduce
surface 13
area
and
surface
energy.
The
ACCEPTED MANUSCRIPT N(CH2CH2OH)3-H+ that derived from the hydrolysis of TEA united with the negative
2
polarity surface (000 1 ) of ZnO pyamid by coulomb interaction. And the
3
N(CH2CH2OH)3-H+ on the (0001) surface as a buffer layer further absorbed growth
4
units. Then another relatively small pyramid grew on the (0001) surface, with the
5
increase of the time of hydrothermal reaction, the small pyramid grew up, then the
6
two pieces of the ZnO nanocrystalline became the same, finally a spindle-like ZnO
7
microstructure was formed.
8
Conclusion
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In this research, one-step hydrothermal method was introduced to synthesize
10
different morphologies of ZnO structures. Compared to microsphere structures,
11
microrod structures, commercial structures and spindle-like structures exhibited
12
outstanding photocatalytic activities due to their unique structures. It was found that
13
the photodegradation of MO followed the pseudo-first-order kinetics. A model was
14
developed to prove that the concentration of TEA played a significant impetus on the
15
growth and forming of the （001） and （001）crystals of ZnO microrods to form the
16
spindle-like structures.
17
Acknowledgements
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This work is financially supported by the Science and Technology Development
19
Plan Project of Shandong Province, China (Grant No.2013GSF11714). The authors
20
declare that they have no conflict of interest.
21 14
ACCEPTED MANUSCRIPT 1
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Figure captions
2
Fig.1 The different photocatalytic degradation curves of MO, without any ZnO
3
sample, with commercial ZnO and with the synthetic samples.
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4 5
Fig.2 Linear relationship of MO solution concentration and absorbance (R2
6
=0.99986).
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Fig.3 The absorbance of MO solution under UV irradiation for different time (a), and
9
the corresponding photos of the solution (b).
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Fig.4 Recycling experiments of visible-light photocatalytic degradation of MO over
12
the spindle-like ZnO microparticles (light source: 500 W of tungsten lamp; catalyst
13
dose: 30 mg; reaction time: 180 min; [MO]: 20 mg/ L; volume 50 mL).
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Fig.5 Relationship between degradation of methyl orange and degradation time: TOC
16
removal efficiency.
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15
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Fig.6 Relationship between ln TOC0/TOCt and irradiation time of different ZnO
19
structures.
20 21
Fig.7 UV–vis absorption spectra of as-prepared ZnO samples.
22 21
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Fig.8 UV–vis absorption spectra of sample C and the tangent curve of the forbidden
2
band width.
3
Fig.9 XRD patterns of the synthesized ZnO (a) with 3mL TEA (b) with 4mL TEA (c)
5
with 5mL TEA (d) with 6 mL TEA and (e) with 7mL TEA.
6
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Fig.10 SEM and TEM images of ZnO prepared (a) with 3mL TEA (b) with 4mL TEA
8
(c) with 5mL TEA (d) with 6mL TEA and (e) with 7mL TEA, (f) one crystal grain of
9
sample C.
10
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Fig.11 HRTEM images of ZnO with 5mL TEA and Selected-area electron diffraction
12
patterns of the same samples.
14 15
Fig.12 EDS image of the 5mL TEA addition ZnO sample.
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11
Fig.13 N2 adsorption–desorption isotherms and pore size distribution curves of
17
spindle-like ZnO (a) and commercial ZnO (b).
18
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16
19
Fig.14 (a) The pencil-like morphology as the ideal morphology of ZnO crystal, (b)
20
(001) surface appears when the surface growth rate along <001> direction is low, (c)
21
{110} surface appears when the surface growth rate along <110> direction is low.
22
ACCEPTED MANUSCRIPT 1
Table legends
2
Table 1 Kinetic parameters for TOC reduction of MO by different morphological ZnO
3
structures.
5
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Table 2 The EDS results of the samples.
6
Table 3 Morphology, particle size, crystallite size and catalytic activity of the three
8
ZnO samples.
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9 10
Table 4 Surface properties of spindle-like ZnO and commercial ZnO.
11
Table 5 The surface growth rates of {100}, {110}, {011}, (001) and (001) of ZnO
13
crystal.
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ACCEPTED MANUSCRIPT Table 1 Kinetic parameters for TOC reduction of MO by different morphological ZnO structures. k (min−1)
R2
Kinetic equation
A (3ml)
In (TOC0/TOCt) = 0.00388t – 0.00934
0.00388
0.99722
B (4ml)
In (TOC0/TOCt) = 0.00417t – 0.00624
0.00417
0.99438
C (5ml)
In (TOC0/TOCt) = 0.01042t – 0.01735
0.01042
0.99532
D (6ml)
In (TOC0/TOCt) = 0.00384t – 0.01604
0.00384
0.99336
E( 7ml)
In (TOC0/TOCt) = 0.00359t – 0.02471
0.00359
0.98861
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ZnO samples
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Table 2 The EDS results of the samples. element
percentage by weight
atomic percentage
C
0.34
1.15
19.72
49.62
79.93
49.23
100
100
O Zn
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total
Table 3 Morphology, particle size, crystallite size and catalytic activity of the three ZnO samples. Particle size (µm)
Crystallite size (nm)
MO solution degradation rate in 180 min (%)
49
62.40
3-5
34
68.47
EP
Morphology
Irregular polyhedron Less microrods， microspheres and Irregular polyhedron
AC C
ZnO sample
2-4
C
Spindle-like
5-8
23
97.67
D
Microspheres and less microrods
3-6
40
65.32
E
Microspheres
4-6
57
55.79
A
B
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Table 4 Surface properties of spindle-like ZnO and commercial ZnO. Total pore volume (cm3/g)
7.93
0.086
spindle-like ZnO commercial ZnO
RI PT
BET surface area (m2/g)
0.003
SC
3.25
crystal.
M AN U
Table 5 The surface growth rates of {100}, {110}, {011}, (001) and (001) of ZnO
Superficial area Ahkl(Å2)
Surface bonding energy S Ehkl（kcal/mol）
Total bonding energy t Ehkl（kcal/mol）
{100}
33.845
83.945
663.221
Relative growth rate of surface Rhkl 1
{110}
55.052
167.891
663.221
1.44
{011}
38.465
125.918
663.221
1.42
(001)
18.291
125.918
663.221
2.99
39.887
663.221
0.82
EP 18.291
AC C
(00)
TE D
Crystal face (hkl)
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Highlights The morphology of ZnO microparticles was spindle-like. A simple one-step hydrothermal method was used to synthesize the specimen.
RI PT
The product presented a high photocatalytic performance and excellent recycle performance.
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The growth mechanism of the spindle-like ZnO microparticles was discussed.