Synthesis of BiOCl Rectangular Nanostructures

BiOCl rectangular nanostructures with tetragonal phase have been successfully prepared via a sodium dodecyl sulphate (SDS)-assisted hydrothermal route using BiCl3 as Bi source material. The products were characterized by X-ray diffraction and scanning electron microscopy. The product exhibits regular rectangular structure with the length, width and thickness of 0.35-1 μm, 0.25-0.75 μm and about 80 nm, respectively. SDS plays an essential role on the formation of the BiOCl rectangular nanostructures. The formation of the BiOCl rectangular nanostructures can be controlled by adjusting hydrothermal temperature, time and SDS concentration. [DOI: 10.1380/ejssnt.2011.297]


I. INTRODUCTION
Bismuth oxychloride (BiOCl) is a wide bandgap semiconductor (E g = 3.46 eV) with layered structures exhibiting good electrical, optical and mechanical properties and promising application potential in cosmetics, pharmaceuticals, battery cathodes, photocatalysis and photoelectrochemical devices [1,2].Because of the unique layered structure and high photocorrosion stabilities in the presence of redox pairs, BiOCl nanostructures has recently been shown as a novel material for photocatalysis, photoelectrochemical cells [3] and exhibits catalytic properties for the oxidative cracking of n-butane to lower alkenes [4].
Several kinds of BiOCl nanostructures have been successfully prepared.Peng et al. [2] reported the synthesis of BiOCl nanostructures with different morphologies, such as nanowires, nanobelts, nanoflowers, nanoflakes and platelets by a vapour phase route on various substrates by thermal evaporation of a AuCl 3 /Bi mixture or BiCl 3 at low temperature.Ma and coworkers [5] synthesized ultrathin BiOCl nanoflakes, nanoplate arrays and curved nanoplates via an ionothermal synthetic route by using an ionic liquid 1-hexadecyl-3-methylimidazolium chloride as "all-in-one" solvent.BiOCl nanoscrystalline and nanosheets have also prepared by solution-phase route from BiCl 3 , ammonia and a facile sonochemical method in a surfactant/ligand-free system under ambient air [6,7].Although well-defined nanostructures of BiOCl have been prepared, it is still a big challenge to develop an alternative route to prepare BiOCl nanostructures with novel morphologies and properties.However, up to date, BiOCl rectangular nanostructures are not yet reported.As widely and efficiently used anionic surfactant, sodium dodecyl sulphate (SDS) has been used as microreactors for the preparation of specific morphologies of nanostructures [8,9].Herein, BiOCl rectangular nanostructures have been prepared by a SDS-assisted hydrothermal process using BiCl 3 as the Bi source material, which provides a facile method for direct hydrothermal growth of specific nanostructures.The roles of hydrothermal temperature, reaction time and SDS concentration on the formation of the BiOCl rectangular nanostructures have been investigated in detail.

II. EXPERIMENTAL
High pure BiCl 3 powder (AR grade, purity: ≥99.0%) and SDS (AR grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. of China.The raw materials were used without further purification.In a typical procedure, 0.6 g BiCl 3 and SDS with the concentration of 0.1 mol/L were dissolved in 60 mL deionized water.The mixed solution was placed in a 100 mL autoclave with a Teflon liner.After heating at 180 • C for 24 h, the autoclave was cooled naturally in air.The resulted white precipitates were filtered and then washed with deionized water for several times.The precipitates were dried at 60 • C in air and finally the white powders were obtained.
The obtained products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).XRD pattern was carried out on a Bruker AXS D8 X-ray diffractometer equipped with a graphite monochromatized Cu-Kα radiation (λ = 1.5406Å).The samples were scanned at a scanning rate of 0.05 • /s in the 2θ range of 20 • -80 • .SEM observation was performed using JEOL JSM-6490LV SEM with a 15-KV accelerating voltage.

III. RESULTS AND DISCUSSIONS
The phase and purity of the products synthesized from 180 • for 24 h were examined by XRD.The XRD pattern of the product is shown in Fig. 1.All the diffraction peaks can be indexed as the tetragonal BiOCl phase (JCPDS card, No. 06-0249).No other phases are detected in Fig. 1 indicating the high purity of the final products.The intense and sharp diffraction peaks suggest that the as-synthesized products are well crystallized.
Figure 2 shows the morphology and size of the products synthesized from 180 • C for 24 h.It is found that the products are composed of many BiOCl rectangular nanostructures.The product exhibits regular rectangular structure with the length, width and thickness of 0.35-1 µm, 0.25-0.75µm and about 80 nm, respectively.The BiOCl rectangular nanostructures are similar to those of Cu 2 O and MgO [10][11][12].
In order to analyze the formation process of the BiOCl rectangular nanostructures, the growth conditions, such as hydrothermal temperature, reaction time and SDS concentration dependences on the formation of the BiOCl rectangular nanostructures are analyzed.The reaction time dependence results on the formation of the BiOCl rectangular nanostructures at 180 • for different time using 0.1 mol/L SDS are shown in Fig. 3.As can be seen, the reaction lasted for 0.5 h, the products are composed of microspheres with nanosheets (Fig. 3(a)).The thickness of the nanosheets in the microspheres is about 100 nm (Fig. 3(b)).When the reaction time was prolonged to 6 h, the products contain a small amount of rectangular structures besides irregular particles, which is shown in Fig. 3(c).This process continued, and more BiOCl rectangular nanostructures formed.When the reaction time was extended to 12 h, most of the products are composed of regular rectangular nanostructures (Fig. 3(d)).
Generally, hydrothermal temperature is considered to have a great impact on the crystal morphologies of final products.Analogous experiments at different temperatures for 24 h using 0.1 mol/L SDS were carried out.The SEM results are shown in Fig. 4. The products are almost nanosheets with the thickness of less than 100 nm as the hydrothermal temperature is 80 • C, which is shown in Figs.4(a) and (b).When the hydrothermal temperature increases to 120 • C, the products are all BiOCl rectangular nanostructures (Figs.4(c) and (d)).The length, width and thickness of the BiOCl rectangular nanostructures decrease to 0.2-0.5 µm, 0.3-0.9µm and about 50 nm, respectively.
According to the present results, it is obviously that BiOCl nanosheets form when the hydrothermal temperature and reaction time decrease to below 80 • C and 6 h, respectively.The formation of the sheet-shaped structure  is generally considered to be a self-assemble process.Suitable templates, such as hard and soft templates are also commonly used [13,14] for the synthesis of BiOCl sheetshaped structure.BiOCl sheet-shaped microspheres have been synthesized using ethylene glycol as the solvent and a soft template by the hydrothermal method ture and reaction time.Therefore, from the shape evolution of BiOCl from different hydrothermal conditions, it is considered that the BiOCl rectangular nanostructures originate from the BiOCl nanosheets.
To substantially understand the effect of SDS on the BiOCl rectangular nanostructures, the experiments using SDS with different concentration were carried out.Figures 5(a nanoparticles spontaneously appear in the supersaturated solution which serve as the nuclei for the formation of the BiOCl nanosheets.The BiOCl nanosheets grow continously with the increase of the hydrothermal temperature and time resulting in the formation of BiOCl sheets with different size.It is well known that usage of a surfactant-assisted reaction to control the nucleation and growth is a simple and effective way.With the increase of hydrothermal temperature, time and SDS concentration, when the SDS is added into the reaction solution, the nuclei and growth proceed slowly owing to the adsorption of hydrophilic group of the SDS microspherical capsules [15].Because of the electrostatic interaction of the sulfonic group and the Bi 3+ , the outer surface of the capsules is occupied by a lot of Bi 3+ cations.Then the absorbed Bi 3+ cations are converted into BiOCl nuclei during the hydrothermal process.When low concentration of SDS, such as 0.05-0.1 mol/L SDS was added into the reaction solution, the insufficient adsorption of hydrophilic group of the SDS microspherical capsules leads to the existence of some crystal faces with high surface energy retarding the crystal growth along the crystal faces.Then the BiOCl rectangular nanostructures form in the reaction solution.When the SDS concentration is 0.2 mol/L in the reaction solution, the excess SDS completely covers the BiOCl particles leading to the homogeneous growth the BiOCl crystals.Therefore, Homogeneous BiOCl nanoparticles with sphere-shaped morphology are formed which is shown in Figs.5(g) and (h).

IV. CONCLUSIONS
In summary, a simple and facile SDS-assisted hydrothermal route has been used to synthesize BiOCl rectangular nanostructures with tetragonal phase using BiCl 3 as the Bi source material.The product exhibits regular rectangular structure with the length, width and thickness of 0.35-1 µm, 0.25-0.75µm and about 80 nm, respectively.BiOCl rectangular nanostructures with different size can be obtained by controlling hydrothermal temperature, reaction time and SDS concentration.When the concentration of SDS in the solution reaches a suitable value, such as 0.05-0.1 mol/L, BiOCl rectangular nanostructures can be produced.The method may be used in the assembly of other nanomaterials with structure similar to BiOCl.

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FIG. 1: X-ray diffraction of the products synthesized from 180 • C for 24 h.

FIG. 2 :
FIG. 2: (a) and (b) SEM images of the BiOCl rectangular nanostructures with different magnifications synthesized from 180 • C for 24 h.
FIG. 4: SEM images of the products synthesized from different hydrothermal temperature for 24 h using 0.1 mol/L SDS.(a) and (b) 80 • C, (c) and (d) 120 • C.
FIG. 5: SEM images of the products synthesized under the hydrothermal conditions of 180 • C for 24 h using SDS with different concentration as the surfactant.(a) and (b) without SDS, (c) and (d) 0.02 mol/L, (e) and (f) 0.05 mol/L, (g) and (h) 0.2 mol/L.

FIG. 6 :
FIG. 6: Schematic illustration of the formation process of the BiOCl rectangular nanostructures in the whole synthetic process.