Coprecipitation synthesis of hollow Zn2SnO4 spheres

Materials Letters 61 (2007) 3005 – 3008 www.elsevier.com/locate/matlet

Coprecipitation synthesis of hollow Zn2SnO4 spheres Shumei Wang, Zhongsen Yang, Mengkai Lu ⁎, Yuanyuan Zhou, Guangjun Zhou, Zifeng Qiu, Shufen Wang, Haiping Zhang, Aiyu Zhang State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China Received 16 March 2006; accepted 30 July 2006 Available online 9 November 2006

Abstract Pure and Dy3+-doped Zn2SnO4 (ZTO) hollow spheres, integrated with nanoparticles, have been synthesized using the coprecipitation method. The ZTO spheres have been characterized with X-ray diffraction and scanning electron microscope. The photoluminescence and photocatalytic properties are investigated. The influences of Dy3+ ions on the morphology and the photoluminescence of ZTO have been explained in detail. © 2006 Elsevier B.V. All rights reserved. Keywords: Zn2SnO4; Photoluminescence; Photocatalytic properties

1. Introduction Transparent conducting oxides ( TCOs) have widespread applications such as in smart windows, flat-panel displays, thinfilm photovoltaic, polymer-based electronics and architectural windows [1–3]. As an important example of TCO materials, the ternary semiconductor Zn2SnO4 ( ZTO) is known for having high electron mobility, high electrical conductivity, and attractive optical properties, all of which make it suitable for a wide range of applications, such as photovoltaic devices and sensors for humidity, combustible gases [4–6]. Quite a number of researches on ZTO have primarily been limited to ZTO thin films [7–10]. In recent years, many efforts have been made to control the sizes and shapes of nanostructures, because these parameters determine their electrical and optical properties [11–15]. Organizing nanoscale building blocks into complex nanostructures is always a target for researchers [16–19]. Although the ZTO nanowires and nanobelts have been prepared in the past two years by using a thermal evaporation method [20,21], the synthesis condition is tied up by many rigid aspects. In this paper, ternary semiconductor Zn2SnO4 hollow spheres were prepared using the simple coprecipitation method without any

⁎ Corresponding author. E-mail address: [email protected] (M. Lu). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.197

surfactant. This method offers several attractive advantages such as: simplicity of the process, availability of mass production and good repeatability with low cost. To the best of our knowledge, this type of ZTO hollow spheres synthesized without any surfactant has never been reported before. Growth of these hollow spheres will open a new option for assembling nanoscale blocks into low-dimensional structure. 2. Experiment All reagents were analytical reagent grade and used without further purification. The zinc nitrate [Zn(NO3)2] solution was obtained by dissolving ZnO in nitric acid. Stannic chloride pentahydrate (SnCl4·5H2O) and Zn(NO3)2 in a 1:2 molar ratio were dissolved in a minimum amount of distilled water. The mixture was added dropwise into ammonia (NH3·H2O) aqueous solution ( VNH2·H2O:VH2O = 2:3) under magnetic stirring. The final solution was alkalescence with a pH value around 8 to make the reagents react completely. The as-synthesized precursor was filtered and washed with distilled water to remove undesirable anions such as Cl− and NO3−, dried at 120 °C for 2.5 h, then calcined in an oven at 1000 °C for 1 h. The crystal phase of the product was determined by X-ray diffraction with CuKα radiation (RigaKu RIN2200). Scanning electron microscopy (SEM) images were performed with a JEOL JSM-6700f scanning electron microscope. The photoluminescent properties were measured with a Hitachi 850

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Fig. 3. Photoluminescent spectrum of the as-synthesized ZTO. Fig. 1. XRD pattern of the as-synthesized product.

fluorescence spectrometer. Photocatalytic activity of the prepared powers was evaluated by the degradation of methyl orange. 3. Results and discussion X-ray diffraction ( XRD) was used to examine the crystal structure and phase purity of the final sample (Fig. 1). From the XRD pattern, most of the sharp diffraction peaks can be assigned well with facecentered spinel-structure Zn2SnO4 ( JCPDS No: 24-1470). The residual peaks marked with black dot symbols can be assigned to SnO2 (JCPDS

No: 41-1445). These peaks of SnO2 are much lower than those of ZTO and much fewer than the standard data of SnO2 phase, indicating that an infrequent amount of SnO2 exists in the as-synthesized sample. The coexistence of SnO2 may be caused by two aspects: 1) the evaporation of a part of ZnO, and 2) thermal decomposition of the assynthesized precursor. Stambolova et al. [9] and Hashemi et al. [22] had indicated that the evaporation of part of ZnO was the present problem for preparing the single phase of Zn2SnO4. On the other hand, ZnSn(OH)6 precursor with a cubic structure will be synthesized after the Zn2+ and Sn4+ solutions were thoroughly mixed under vigorous stirring at room temperature. The final product, Zn2SnO4 with an infrequent amount of SnO2, is obtained by thermal decomposition of ZnSn( OH)6 precursor. The reactions for formation of Zn2SnO4 can be summarized as follows: 2Zn2þ þ 2Sn4þ þ 12OH− →2ZnSnðOHÞ6 ↓→Zn2 SnO4 þ SnO2 þ 6H2 O↑ It is hard to detect the diffractions according to the phase with a 1:1 cation ratio (ZnSnO3) under the present synthetic procedure. This may be due to the fact that the only stable phases at high temperature (N750 °C) in the ZnO–SnO2 system are Zn2SnO4, ZnO and SnO2, and the thermal decomposition of ZnSn(OH)6 can only yield amorphous ZnSnO3 stable within the temperature range 350 °C–750 °C [23]. The morphology of the synthesized product was indicated with the typical SEM images (Fig. 2a and b). The SEM images show that the

Fig. 2. SEM images with different magnifications of ZTO hollow spheres.

Fig. 4. Results for photocatalytic activity of ZTO to methyl orange.

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Fig. 7. Influence of Dy3+ ions on 405-nm emission of ZTO. 3+

Fig. 5. SEM image of 2 mol% Dy -doped ZTO.

product consists of a large quantity of capsules. These capsules are integrated with nanoparticles within 100 nm and not easily fragmented into pieces (Fig. 2a). As seen from the mesopores on the coarse surface of the capsules (Fig. 2b), it is reasonably to think that these capsules are hollow. The sizes of the hollow spheres are around 300 to 500 nm with average size of 400 nm. ZnSn(OH)6 precursor is a cubic phase with a space group of Pn3¯ (201) (JCPDS NO: 73-2384). In this work, under violent conditions (i.e. high temperature), the metastable cubic form, bound by the less stable {100} planes, will transform into a highly stable spherical form. That is to say, during the high temperature calcination process, H2O molecules are continuously produced and released as a gaseous form from the matrix cubes, leaving crystallized Zn2SnO4 composites with hollow structure behind. This progress has also been confirmed by Lu et al. [24]. As the proceeding of the heat-treatment, some small particles grow larger at the expense of smaller grains. The photoluminescent spectrum of the as-synthesized ZTO was measured at room temperature, as shown in Fig. 3. When excitated at 300 nm, the product exhibits two emission peaks around 360 nm and 405 nm, respectively. It has been reported that the optical band gap of the bulk ZTO is ∼ 3.6 eV [25]. The emission at 360 nm is obviously the band-to-band emission peak. In the previous investigations of the semiconductor nanomaterials, the PL mechanisms have always been attributed to other luminescent centers, such as oxygen vacancies and residual strain during the growth progress [26–28]. In our experiment, 1 h heating at 1000 °C was used to synthesize the ZTO hollow spheres. The oxygen vacancies are inevitably generated because of the insufficient oxygen in the calcination progress. In addition, the transformation from

metastable cubic form to a highly stable spherical form favors the existence of large quantities of oxygen vacancies. These oxygen vacancies induce the formation of new energy levels in the band gap of the ZTO hollow spheres. The emission thus results from the recombination of a photo-excited hole with an electron occupying an oxygen vacancy. For these deeper energy levels, the emission moves to a longer wavelength, and the 405-nm emission is observed. To fully understand the detailed luminescent mechanism, more systematic investigations are required. Wang et al. [29] had studied the photocatalytic decomposition behavior of benzene in the water solution using ZTO as photocatalyst. In the present study, Zn2SnO4 has been used as photocatalyst to decompose methyl orange in water solution. The results of the blank experiment for photocatalytic activity of ZTO to methyl orange are displayed in Fig. 4. After the methyl orange solution was magnetically stirred with Zn2SnO4 for 0.5 h without UV irradiation, the absorbency (the dashed line) of the as-tested solution scarcely changes compared with that of the pure methyl orange solution (not shown). This phenomenon eliminates the adsorption of Zn2SnO4 to methyl orange. The absorbency, however, decreases from 1.618 to 1.395 at 464 nm after the above mixture was magnetically stirred for 0.5 h with UV irradiation (the solid line), suggesting the photocatalytic activity of Zn2SnO4 to methyl orange. The decolorization rate (η) is 13.78%, which can be calculated with the formula [η = (Ao − A) / Ao × 100%], where Ao and A stand for the absorbency of the undecomposed and decomposed solution, respectively. The influences of Dy3+ ions on the morphology and photoluminescence of Zn2SnO4 were also investigated in the present work. The Dy3+-doped Zn2SnO4 with the doped concentration from 0.5 mol%

Fig. 6. Schematic model for the calcining of pure and doped crystalline ZTO.

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to 2 mol% were crystallized by using the same synthesis method. The SEM image of the 2 mol% Dy3+-doped Zn2SnO4 is presented in Fig. 5. The SEM image shows that the sizes of the capsules are around 100 nm to 300 nm with average size of 200 nm, which is smaller than that of pure Zn2SnO4. On the other hand, most particles integrating themselves into capsules are much larger than those of pure Zn2SnO4. A small quantity of small particles, which are much smaller than those of pure Zn2SnO4, is located tightly on the surfaces of the capsules. A simple pictorial model of the calcining behavior of ZTO is shown in Fig. 6. For pure ZTO, the hollow spheres are obtained with the forenamed progress. For Dy3+-doped ZTO, the initial stage of calcining is similar to that of pure ZTO. However, the surface of the grains (the broken particles) is likely to have a dysprosium character, which is further amplified by segregation during the initial stage of calcining. The grain growth stops because the mobility in the dysprosiumdominated interfaces (grain boundaries) is low, which causes the grain sizes of Dy3+-doped ZTO to be much smaller than those of pure ZTO. The similar condition occurs in the Al3+-doped ZrO2 [31]. The small grains have a strong aggregation to grow into the single large grains because of the high surface energy, and integrate themselves to form the capsules of ZTO. The traces on the surface of the large grains and the remnant small grains located tightly on the surfaces of the large grains explain the change before and after aggregating. These formation processes make either the hollow spheres or the corresponding cavities of Dy3+-doped ZTO small. The band-to-band emission of ZTO is not influenced by the addition of Dy3+ ions (not shown). Fig. 7 describes the luminescent intensity curve of 405-nm emission basing on Dy3+ concentrations. The increase of the intensity is due to an efficient energy transfer from doped ions to the ZTO host in Dy3+-doped Zn2SnO4. The decrease of the luminescent intensity above the optimum concentration, i.e. 1 mol% Dy3+, can be attributed to the concentration quenching.

4. Conclusions i. The Zn2SnO4 hollow spheres, integrated with nanoparticles, can be synthesized using the coprecipitation method with proper reaction conditions. The initial nanoparticles within 100 nm are formed by the transformation from the metastable cubic form, bound by the less stable {100} planes, into a highly stable spherical form. ii. There are two emission peaks in the photoluminescent spectrum of the as-synthesized Zn2SnO4, which can be attributed to the band-to-band emission and the oxygen vacancies defect emission, respectively. iii. Zn2SnO4 has photocatalytic activity to methyl orange under UV irradiation. The decolorization rate (η), calculated with the formula, is 13.78%. iv. The addition of Dy3+ ions can stop the grain growth of ZTO and make the grain small to aggregate the single large grain, which integrates further to form small capsules of ZTO. The addition of Dy3+ ions has no influence on the band-to-band emission of ZTO, but can

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