One-step synthesis of novel hierarchical flower-like SnO2 nanostructures with enhanced photocatalytic activity

Accepted Manuscript One-step synthesis of novel hierarchical flower-like SnO2 nanostructures with enhanced photocatalytic activity Xiangyu Chen, Deqing Chu, Limin Wang, Wenhui Hu, Huifang Yang, Jingjing Sun, Shaopeng Zhu, Guowei Wang, Jian Tao, Songsong Zhang PII:

S0925-8388(17)32823-2

DOI:

10.1016/j.jallcom.2017.08.094

Reference:

JALCOM 42851

To appear in:

Journal of Alloys and Compounds

Received Date: 6 April 2017 Revised Date:

0925-8388 0925-8388

Accepted Date: 10 August 2017

Please cite this article as: X. Chen, D. Chu, L. Wang, W. Hu, H. Yang, J. Sun, S. Zhu, G. Wang, J. Tao, S. Zhang, One-step synthesis of novel hierarchical flower-like SnO2 nanostructures with enhanced photocatalytic activity, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.08.094. 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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One-step synthesis of novel hierarchical flower-like SnO2 nanostructures with enhanced photocatalytic activity Xiangyu Chen,a,b Deqing Chu,*,a,b Limin Wang,*,a,c Wenhui Hu,b Huifang Yang,b Jingjing Sun,c Shaopeng Zhu,b Guowei Wang,c Jian Tao,b Songsong Zhang,b a

b

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State Key Laboratory of Separation membranes and Membrane Processes, Tianjin 300387, PR China. College of Environment and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China.

c

School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China.

*Corresponding author. Tel: +86-022-83955762; Fax: +86-022-83955762;

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E-mail: [email protected]; E-mail: [email protected]

Abstract

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Novel hierarchical flower-like SnO2 (HFL-SnO2) nanostructures are successfully synthesized through the hydrothermal method. We find that the hierarchical architectures of SnO2 is assembled by a number of regular-shaped nanosheets, which are cross-linked in the interior

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of the micro-flower. The product also has a high specific surface area of about 78.8 m2g-1. Based on the nucleation and self-assembly

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process, a possible formation mechanism is proposed. The effects of SDS and NaCl on the product are also studied. Moreover, the

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product exhibits excellent photocatalytic activity and take possession of good stability and reusability under visible light. This study provides

valuable experimental

data

for

further

study

of

nanomaterials and their photocatalytic properties. Keywords: SnO2; flower-like; photocatalytic activity; Rhodamine B; 1. Introduction

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The hierarchical self-assembly of nano/micromaterials with specific morphologies in the chemical field is of great interest to the materials science because their complex structures tend to provide

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some unique and exciting properties [1-3]. However, the synthesis of such morphological materials generally involves many complex procedures, harsh reaction conditions, especially very small amounts

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of the product. Therefore, the development of synthetic materials

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with large-scale synthesis of high quality similar to oxide nanocrystals, and the use of green synthetic materials has attracted more and more research interest.

As an important multifunctional n-type wide band gap

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semiconductor oxide (Eg= 3.6eV, at 300K), SnO2 has attracted widely attention in basic research and practical applications, such as photocatalyst [4], gas sensor [5], solar cell [6] and electrode material

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[7]. In the past few years, various geometries of SnO2 have been

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reported, such as nanorods [8], nanowires [9], nanobelt [10], and nanoflowers [11]. Compared to low dimensions (0D, 1D and 2D), 3D hierarchical nanostructures have many unique advantages, such as low density, high porosity, high specific surface area and increased surface-to-volume ratio, which is beneficial to improve the performance of SnO2 [12]. Among them, SnO2 three-dimensional (3D) structure due to its excellent photocatalytic activity cause

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widespread attention [13]. However, so far, there is no research has been reported on the synthesis of flower-like SnO2 (HFL-SnO2) nanostructure and its photocatalytic performance.

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Currently, water pollution has become a direct threat to human survival, the urgent need to solve the problem. In the control of water pollution, dyeing wastewater because of its variety, toxic,

become

difficult

task

in

sewage

treatment.

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characteristics,

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heavy environmental pollution and difficult to degrade and other

Photocatalytic technology can be the degradation of organic pollutants in dye wastewater into non-toxic or low toxicity easily degraded small molecules. It is efficient, energy saving, no

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secondary pollution. Therefore, the preparation of excellent photocatalyst has become a key issue in the treatment of dye-containing wastewater.

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In this paper, we report a one-pot and facile method for

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synthesizing novel hierarchical flower-like SnO2 nanostructures by hydrothermal technique. The nanostructures and personal properties of the product are investigated by using XRD, SEM, XPS and BET. Combined with time-dependent experiments and characterization, we present a possible growth mechanism for the product. In addition, the HFL-SnO2 nanostructures exhibit high catalytic activity for degradation of RhB under visible light and also have good stability

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and reusability for RhB. 2. Experiment 2.1. Chemicals and synthesis of the samples

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None of the reagents in the experiments are further purified. In a typical synthetic method, 1mmol SnCl2·2H2O, 0.03 g SDS and 0.12 g of NaCl are dissolved in 10 ml of distilled water and stirred for 15

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minutes to form solution A. 5 mmol of NaOH is added to 10 ml of

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distilled water to form solution B. Solution B is added slowly to solution A under magnetic stirring for 20 minutes. The precursor solution is then transferred to a 20mL Teflon-lined stainless steel autoclave and hold at the temperature of 160

for 6h. The resulting

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precipitate is centrifuged, rinsed with absolute alcohol and distilled water, and dried in vacuo at 60 . 2.2. Photocatalysis measurement

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Photocatalytic degradation of RhB is carried out in aqueous

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solution at room temperature. In a typical experiment, the prepared HFL-SnO2 nanostructures (100mg) are dispersed in 100mL of RhB aqueous solution at a concentration of 20mg L-1. Before illumination, the suspension is stirred in the dark for 30min to obtain an adsorption analytical equilibrium of the RhB molecules. Then, the mixed system is irradiated with visible-light. During the whole irradiation process the suspension remained stirring. At regular

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intervals, approximately 3mL of reaction solution is sampled and analyzed. The concentration of RhB in the degradation process is detected by a UV-vis spectrophotometer measurements at 552nm.

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The degradation rate of RhB is recorded as (C0-C)/C0, where C0 is

any sampling time. 3. Result and discussion

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3.1. Structure and morphology

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the concentration before irradiation, and C is the concentration at

The SEM image provides an understanding of the morphology of HFL-SnO2 nanostructures. The enlarged SEM image shows the close observation of the SnO2 nanostructures in Fig. 1(a). It reveals a

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detailed and clear nanostructure of flower-like SnO2 products with a diameter of about 1 µm. The HFL-SnO2 nanostructures are formed by nucleation of nanosheets. The probably morphology of the

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HFL-SnO2 nanostructures is shown in Fig. 1(b), indicating high

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yields and good uniformity. Meanwhile, we carried out XRD characterization of novel flower-like SnO2 nanostructures. As shown in Figure 1(c), all peaks of the HFL-SnO2 nanostructures are well matched to standard data (JCPDS card number 41-1445) [14]. In addition to these SnO2 peaks, no other peaks are observed, indicating the high purity of the SnO2 product. The strong and sharp reflection of the peak indicates that the product is highly crystalline. We also

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use EDS to characterize the HFL-SnO2 nanostructures. From Figure 1(d) we can know the Sn element, O element ratio of 1 : 2, further proof the product is SnO2. the

surface

property

of

the

HFL-SnO2

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Furthermore,

nanostructures was studied by XPS spectroscopy. Fig. 2 (a and b) displays the wide XPS spectrum of the as-prepared SnO2 product.

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There is no extra peaks except Sn and O observed, indicating the

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high purity of SnO2. Fig. 2a shows the high-resolution XPS spectra for Sn3d. The peaks sitting at 487.5eV and 495.9eV for Sn3d region are attributed to Sn 3d5/2 and Sn 3d3/2, respectively. Fig. 2b shows the O 1s spectrum. The BE at 531.4eV corresponding to O 1s indicates that the oxygen atoms exist as O2 species in the compounds. From

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the XPS measurements, it was determined that the synthesized product was pure SnO2 [21]. All these species were confirmed from

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XRD, EDS and XPS. Thus, we concluded that the HFL-SnO2

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nanostructures was successfully synthesized. 3.2. Formation mechanism Fig. 3 shows the SEM images of a sample obtained by different

hydrothermal reaction times. When the hydrothermal reaction begins, the reaction system has crystal nuclei because the system has a large surface energy. At this point, the nucleus grows rapidly into SnO2 particles. In order to reduce the surface energy of the system, tin

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oxide particles self-assembled together to form a sheet of nanostructures. When the reaction was carried out for 3 hours (Fig. 3(a)), the sheet-like nanostructures were grown in different

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directions and gradually self-assembled into irregular flaky hybrid. When the hydrothermal time lasts up to 6 hours (Fig. 3(b)), the self-assembled structure begins to grow uniformly and orderly.

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Prolonged to 8 hours (Fig. 3(c)), with the continuation of the

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reaction, the formation of more SnO2 further promoted the growth of the flower-like nanostructures, and the petals in the flower-like nanostructures continued to increase. When reacted to 12 hours, the perfect hierarchical epiphyllic-like SnO2 nanostructures are formed

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(Fig. 1(a)). At this point it is the most complete morphology, and the most uniform. Micro-flower flowers and petals have been adjusted to the most perfect state. When the extension time to 18 hours (Fig.

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deform.

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3(d)), flower-like SnO2 nanostructures began to decompose and

Based on the above observations and analysis, we propose a

possible growth mechanism for the formation of novel flower-like nanostructures. When adding distilled water to SnCl2·2H2O, Sn2+ is hydrolyzed and oxidized in water, although a small part of it oxidized to produce Sn(OH)4, but most of them are subsequently oxidized to SnO2 during hydrothermal process [15]. When NaOH

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solution is added to the above mixture, the amphoteric Sn(OH)4 pretreatment immediately becomes Sn(OH)62- ion in an alkaline environment. Sn(OH)4 is continuously consumed to stimulate the

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reaction takes place with the aid of SDS:

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reaction to produce more Sn(OH)4 formation and the following

Then, under the induction of hydrothermal conditions, the initially

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formed SnO2 small crystals gradually grow into larger nanocrystals and begin nucleation. SnO2 nanosheets were formed in order to reduce the surface energy under the driving force of directional growth. The different orientations of the nanocrystals begin to favor

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the random orientation of the nanosheets. And with the extension of the hydrothermal time, the reaction entered the nucleation stage. The

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nucleus began to grow in scale with the help of SDS, followed by growth of the hierarchical nanostructures. Finally, a beautiful

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flower-like SnO2 nanostructures are synthesized successfully. Throughout the reaction, NaCl plays an important role in the

formation of HFL-SnO2 nanostructures. NaCl was rarely used in the synthesis of flower-like SnO2 nanostructures, which is of great significance for future research. So according to the amount of NaCl added to do a comparison, as shown in Figure 4. In our reaction system, one functions of the salt in the reaction system is to increase

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the chemical potential of the solution, and the potential in the highly chemically grown solution favors the growth of nanocrystals. Secondly, the addition of salt to the reaction system significantly

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reduces the viscosity of the solution and increase the mobility of the components in the system. Faster ion motion usually ensures a reversible path phase between the fluid phase and the solid, which

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allows atoms, ions or molecules to take the correct position in the

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lattice development [22]. In addition to NaCl, SDS also has a very important impact on the morphology of the product. In the course of the experiment, the anionic surfactant SDS plays an important role in the formation of SnO2 nanosheets. The presence of SDS results in

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a decrease in the surface tension of the solution, which reduces the energy required to form a new stage. The effect of specific SDS on the morphology is shown in Fig. 5. In the absence of SDS, SnO2 is a

effectively

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surfactant

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reunion structure that is clustered together by nanosheets. SDS as a

nanostructures

during

prevents the

the

agglomeration

formation

of

of

flaky

flower-like

SnO2

nanostructures. It promotes the SnO2 flaky structure to grow in different directions, so that the shape of the product is similar to flower-like structure has been successfully synthesized. 3.3. Photocatalytic properties In order to evaluate the photocatalytic activity of the products, we

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measured the optical properties of RhB aqueous solution in the presence of synthesized HFL-SnO2 product under irradiation of visible light for a given time, and the results are shown in Fig. 6.

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When the solution is irradiated for 120 min in the presence of HFL-SnO2 nanostructures, the supernatant removed is almost no color. The degradation rate of RhB was 93.6% by processing the

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data. It is generally accepted that the catalytic process is mainly

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related to the adsorption and desorption of molecules on the surface of the catalyst. The BET surface area of the SnO2 product is calculated to be 78.8 m2 g-1 (Fig. 7), which is much higher than many of the other SnO2 products that have been reported. The high

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specific surface area of the product results in more unsaturated surface coordination sites exposed to the solution. The flower-like nanostructures in the catalyst enable storage of more molecules.

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Therefore, it nature provides more active reaction sites. In addition, a

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rough comparison of photodegradation efficiency of RhB for various SnO2 catalysts is also listed in Table 1. Combined with other SnO2 materials,

the

HFL-SnO2

nanostructures

exhibit

enhanced

photodegradation activity of RhB under visible light irradiation. We also test the photocatalytic degradation of RhB in the case of UV-light irradiation. It is found that only about 72.5% of the RhB is degraded. In contrast, under the same conditions, we added

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commercial TiO2 and did not add any catalyst to measure the degradation of RhB under the visible light irradiation (Fig. 6(a)). The stability and reusability of the HFL-SnO2 nanostructures were

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examined by repetitive use of the catalyst. As shown in Fig. 6(b), the catalyst did not exhibit a significant loss of activity after four photo-degradation cycles of RhB under the visible light irradiation.

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The loss of the product is partly due to the incomplete collection of

4. Conclusion In

summary,

nanostructures

are

novel

hierarchical

successfully

synthesized

flower-like via

a

SnO2 one-pot

method by carefully optimizing the reaction

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hydrothermal

the

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centrifugal processes.

parameters. The simple synthesis approach casts new light on the controllable fabrication of the SnO2 product. Furthermore, the

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photocatalytic activity of the prepared SnO2 sample is evaluated for

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the photodegradation of RhB under visible light irradiation. It exhibits excellent photocatalytic activity and is found stable and easy to separate after photocatalysis.

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Figure captions:

Fig. 1. SEM images, XRD and EDS pattern of the as-synthesized HFL-SnO2 nanostructures: (a) a detailed image of an individual; (b) overall product morphology; (c) powder XRD patterns of the

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product and (d) EDS patterns of the as-synthesized SnO2 product. Fig. 2. XPS spectra of the as-synthesized HFL-SnO2 nanostructures:

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(a) XPS spectra of Sn3d region; (b) XPS spectra of O1s region. Fig. 3. SEM images of the SnO2 samples obtained after solvothermal

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treatment at different time: (a) 3h;(b) 6h;(c) 9h; and (d) 18h. Fig. 4. SEM images of SnO2 samples obtained with different NaCl additions: (a) 0g; (b) 0.03g; (c) 0.06g; (d) 0.2g. Fig. 5. SEM images of SnO2 samples obtained with different SDS additions: (a) 0g; (b) 0.01g; (c) 0.02g; (d) 0.04g. Fig. 6 (a) Photocatalytic degradation of RhB for the samples; (b) Recycled degradation of RhB for product under visible light

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irradiation. Fig. 7. N2 adsorption–desorption isotherm and BJH pore size distribution plots (inset) of the HFL-SnO2 nanostructures.

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Table caption Table 1: A rough comparison of photocatalytic properties among

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different SnO2 catalysts.

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Table 1

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Highlights ► Novel hierarchical flower-like SnO2 nanostructures are successfully fabricated with a one-step hydrothermal route. ► A possible growth mechanism for novel hierarchical flower-like SnO2 nanostructures is proposed. ► The product has a high specific surface area of about 78.8 m2g-1. ► NaCl was first used to assist in the synthesis of flower-like SnO2 nanostructures. ► The product exhibits excellent photocatalytic activity in the catalytic degradation of RhB and moreover, shows good stability and reusability for RhB under visible light.