From Bulk Metal Bi to Two-Dimensional Well-Crystallized BiOX (X = Cl

Jul 10, 2008 - Thus, exploring new routes to large-scale synthesis of well-crystallized BiOX. *To whom correspondence should be addressed. E-mail: tan...
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From Bulk Metal Bi to Two-Dimensional Well-Crystallized BiOX (X ) Cl, Br) Micro- and Nanostructures: Synthesis and Characterization Zhengtao Deng,†,‡ Dong Chen,† Bo Peng,†,‡ and Fangqiong Tang*,†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2995–3003

Laboratory of Controllable Preparation and Application of Nanomaterials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed January 30, 2008; ReVised Manuscript ReceiVed May 5, 2008

ABSTRACT: In this article, for the first time, two-dimensional (2D) single-crystalline bismuth oxyhalides (BiOX, X ) Cl, Br) micro- and nanostructures, such as nanoplates, nanosheets, and microsheets, were synthesized in a large scale by a simple wet chemistry approach of hydrogen peroxide (H2O2) direct oxidation of bulk metal bismuth (Bi) particles in a mixed solution followed by a hydrothermal treatment, instead of previous coprecipitation of Bi salts route. The in-plane size and thickness of the 2D products can be conveniently tailored by varying the temperature and the concentrations of the Bi precursor. The products were characterized by a range of methods, such as X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, highresolution transmission microscopy, energy-dispersive X-ray spectroscopy, selected area electron diffraction, thermogravimetric analysis, Fourier transform infrared spectra, and UV-vis diffuse reflectance spectra. The BiOBr nanoplates, nanosheets, and microsheets have also been selectively synthesized via a similar route. The formation process investigation revealed that under hydrothermal treatment the spherical Bi oxhydrohalide nanopariticles could be side-by-side self-assembled to form Bi oxyhalide nanoplates by increasing the in-plane size, and nanosheets could be piled up to form Bi oxyhalide thick microsheets by increasing the thickness. The UV-vis diffuse reflectance spectra revealed that the estimated band gap energies were about 3.5, 3.3, 2.3, and 2.1 eV for BiOCl nanoplates, BiOCl micro- and nanosheets, BiOBr nanoplates, and BiOBr micro- and nanosheets, respectively. It is expected that the present study could be extended to facile, large-scale synthesis of various multicomponent 2D inorganic micro- and nanostructures, which would have better performances than the corresponding spherical nanoparticles and would be the new members in the family of advanced functional inorganic materials well-applied in industry. Introduction In recent years, two-dimensional (2D) nanostructured materials, such as nanoplates and nanosheets, have attracted much attention because of not only their unique electronic, magnetic, optical, and catalytic properties, which mainly arise from their large surface areas, nearly perfect crystallinity, structural anisotropy, and quantum confinement effects in the thickness, but also their potential uses for building blocks for advanced materials and devices with designed functions in areas as diverse as lasers, transistors, catalysis, solar cells, light emission diodes, and chemical and biological sensors.1–6 While the synthesis of spherical nanoparticles and nanocrystals,7,8 as well as the onedimensional nanorods and nanowires,9,10 has been successful in many cases, systematic control of the size of nanoplates and nanosheets, that is, the control of crystal growth in two dimensions, has shown to be much more difficult.11,12 In addition, although a few successes have been achieved in the fabrication of nanoplates and nanosheets made of noble metals,13,14 oxides,15,16 and hydroxides,17 it is still a challenge to explore new approaches to tailor the dimensionality and geometry of multicomponent inorganic compound nanoplates and nanosheets with well-defined shapes, good crystallinity and high yield by an environmentally friendly and cost-efficient route. Bismuth oxychlorides (BiOCl) and bismuth oxybromides (BiOBr) belong to the family of main group multicomponent metal oxyhalides V-VI-VII, an important class of ternary * To whom correspondence should be addressed. E-mail: tangfq@ mail.ipc.ac.cn. † Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.

compound because of the coexistence of unique and excellent electrical, magnetic, optical, and luminescent properties.18,19 For example, BiOCl shows the properties of photoluminescence and thermally stimulated conductivity and has industrial applications as pigments in the cosmetic industry.20 BiOCl is also known to have good catalytic activity and selectivity in the oxidative coupling of the methane (OCM) reaction,21–23 and its activities may be enhanced using 2D nanoplates and nanosheets of BiOCl because of their large surface-to-volume ratios. Controlled synthesis of BiOX nanoparticles is also crucial for the better dispersibility of pigments and enhancement of performance.24,25 Therefore, the development of convenient strategies for their preparation 2D nanostructured bismuth oxyhalides is deemed necessary. Several synthesis methods for micro- and nanostructures of BiOX materials have been reported in the literature.24–30 For example, Geng et al. reported a procedure for the one-step selective synthesis of 2D BiOCl lamellae materials via a sonochemical method.25 Henle et al. reported the size controllable synthesis of spherical BiOX (X ) Cl, Br, I) nanoparticles with diameters of 3-22 nm using an effective reverse microemulsions route.26 Deng et al. showed that bismuth oxychlorides nanobelts and bismuth oxybromides nanobelts/nanotubes could be selectively synthesized in a similar method.28 However, most of the reported procedures for BiOX micro- and nanostructures take bismuth (Bi) salts [such as Bi(NO3)3 · 5H2O, BiCl3] as the starting materials by a coprecipitation process, and the products with well-defined 2D morphologies and with well-crystallized micro- and nanostructures are hard to obtain. Thus, exploring new routes to large-scale synthesis of well-crystallized BiOX

10.1021/cg800116m CCC: $40.75  2008 American Chemical Society Published on Web 07/10/2008

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Scheme 1. Illustration of Large-Scale In-Plane Size and Thickness Controllable Synthesis of Single-Crystalline BiOX (X ) Cl, Br) Micro- and Nanostructures (Top) and the Experimental Parameters and the Description of the Products Described in the Scheme (Bottom)

micro- and nanostructures with controllable morphologies is still a great challenge. Herein, we report a new solution route to controlled synthesis of 2D single-crystalline BiOX (X ) Cl, Br) nanoplates, nanosheets, and microsheets in a large-scale by hydrogen peroxide (H2O2) oxidation of bulk metal Bi particles in a mixed solution made of sodium chloride (NaCl)/sodium bromide (NaBr), poly(vinylpyrrolidone) (PVP), and deionized water (DIW) and followed by hydrothermal treatments, instead of the previous coprecipitation of Bi salts.20,23–26 The advantages of the present protocol are the following: (i) Well-crystallized BiOCl and BiOBr nanoplates with ideal crystallinity and welldefined 2D structures have been obtained; these nanostructures of Bi oxyhalides are not reported in the literature. (ii) Our route takes bulk metal Bi and hydrogen peroxide as the source materials, instead of the coprecipitation of Bi salts, which is especially appealing because of they are environmentally friendly and cost-efficient, and it may open a general avenue for the large-scale production of various multicomponent metal oxyhalides 2D nanostructured materials. (iii) The in-plane size and thickness controllable synthesis of 2D single-crystalline nanoplates, nanosheets, and microsheets have been successfully achieved. (iv) The band gap energies of the samples with different sizes were estimated by UV-vis diffuse reflectance spectra, and the band gap blue shifting due to the size-dependent effect is observed.

adding DIW and transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was maintained at a temperature of 160 °C

Experimental Section Scheme 1 illustrates the in-plane size and thickness controllable synthesis of single-crystalline BiOX (X ) Cl, Br) nanoplates, nanosheets, and microsheets. All of the chemical reagents used in this experiment were analytical grade. In a typical experiment, 210 mg of Bi particles (99.99%, e100 mesh), 400 mg of PVP (MW 30000), 120 mg of NaCl, and 2 mL of H2O2 (36% in v/v) were added into 48 mL of DIW. Then, the mixed solution was stirred at room temperature for 48 h. After the reaction, the Bi particles were dissolved, and the mixture was turned into a uniform white suspension. For the synthesis of BiOCl singlecrystalline nanoplates, 5 mL of suspension was diluted to 40 mL by

Figure 1. (a) SEM image of the metal Bi source materials; (b) powder XRD pattern the metal Bi source materials.

BiOX (X ) Cl, Br) Micro- and Nanostructures

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Figure 2. Powder XRD patterns of the as-synthesized small BiOCl nanoplates (a), the as-synthesized thin BiOCl nanosheets (b), the assynthesized thick BiOCl microsheets (c), and the reference pattern (vertical bars) of the JCPDS cards (Card No. 85-0861).

Figure 4. (a) Low magnification TEM image of the as-synthesized BiOCl nanoplates; (b) TEM image of the one typical BiOCl nanoplate; (c) typical HRTEM image of the edge of the nanoplate; (d) typical HRTEM image of the center of the nanoplate; and (e) corresponding SAED pattern of image d. products. The final products were then dried in air at 60 °C for 4 h before characterization. For synthesis of BiOCl nanosheets, 10 mL of suspension was diluted to 40 mL, and the autoclave was maintained at a temperature of 180 °C for 10 h. For synthesis of BiOCl microsheets, 20 mL of suspension was diluted to 40 mL, and the autoclave was maintained at a temperature of 200 °C for 10 h. The BiOBr nanoplates, nanosheets, and microsheets were prepared following the same procedure described above for BiOCl, except using NaBr instead of NaCl. X-ray powder diffraction (XRD) measurement was employed using a Japan Regaku D/max γA X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.5418 Å) irradiated with a scanning rate of 0.02 °/s. Samples for XRD measurement were prepared by carefully dropping the concentrated alcohol solution of product onto the amorphous glass slides and then allowing the solvent to evaporate at room temperature, and the process was repeated several times to obtain the thick films. Scanning electron microscopy (SEM) and energydispersive X-ray spectroscopic (EDX) measurements were preformed using a Hitachi S-4300 scanning electron field emission microscope operating at 15 kV. Transmission electron microscopy (TEM), highresolution TEM (HRTEM), and selected area electron diffraction (SAED) patterns were performed on a JEOL JEM-2010 electron microscope operating at 200 kV. Thermogravimetric analysis (TGA) was conducted on a DuPont Q50 thermogravimetric analyzer from 30 to 900 °C under a heating rate of 10 °C/min and a nitrogen flow with 60 mL/min. Fourier transform infrared (FTIR) spectra were recorded with a Varian 3100 FT-IR spectrometer at room temperature. UV-vis diffuse reflectance spectra were recorded at room temperature with the JASCO 570 spectrophotometer equipped with an integrated sphere. Figure 3. (a, b) Low magnification and (c-f) high magnification SEM images of the as-synthesized BiOCl nanoplates; (g) EDX pattern of BiOCl nanoplates shown in panel b; and (h, i) histogram of the thickness and in-plane size distribution of BiOCl nanoplates (more than 100 nanoplates were measured). for 10 h without stirring and shaking. The resulting solid products were filtered and washed with distilled water to remove residual ions in the

Results and Discussion The source materials were determined with SEM and XRD. Figure 1a shows the typical SEM image and XRD pattern of the source materials of metal bulk Bi particles, which revealed that the sizes of the irregular shaped Bi particles ranged from

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Figure 5. (a, b) SEM and (c, d) TEM images of the as-synthesized BiOCl nanosheets; and (f) typical HRTEM image of the edge of one typical nanosheet. Inset (d, e): the corresponding SAED patterns.

50 to 200 µm. Figure 1b shows the typical XRD patterns of the source material, where all of the diffraction peaks of the source material can be readily indexed as a highly pure metal Bi phase (JCPDS Card No.77-1173). Figure 2 shows the typical XRD patterns of the as-synthesized 2D BiOCl nanostructures, such as nanoplates, nanosheets, and microsheets. The experimental XRD profile taken from the assynthesized BiOCl samples shows that all of the peaks may be indexed as the tetragonal phase BiOCl (cell constants a ) 3.89 Å, c ) 7.37 Å; JCPDS Card No. 85-0861). No peaks of metal

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Bi or any other phases were detected, indicating that the products are very high-purity, single-phase samples. In addition, the intense and sharp diffraction peaks suggest that the assynthesized products are well-crystallized. Furthermore, as seen from the XRD pattern of the as-synthesized nanoplates and nanosheets, the patterns of the {001} facets family, such as (001), (002), and (003), appeared, and other planes, such as (101) and (102), could be omitted relative to bulk tetragonal phase BiOCl, indicating that the basal planes of the nanosheets should be dominated by {001} facets; therefore, their {001} planes tend to be preferentially oriented parallel to the surface of the substrate in the XRD experiment. For the XRD pattern of the BiOCl microsheets, the {001} facets family, such as (001) and (002), peak intensities are significantly enhanced, and the (101) and the (102) peak intensities are significantly decreased relative to bulk tetragonal phase BiOCl, indicating the preferred crystallographic orientation of the product. With the method of preparation of the XRD samples as described in the Experimental Section, the nanoplates and nanosheets samples are coincidentally oriented in the same direction due to their large in-plane size to thickness ratios; thus, their XRD patterns show only the {001} facets family. However, the XRD pattern of the microsheets shows (101) and (102) planes besides the {001} facets family, which is due to the decreased in-plane size to thickness ratio of the microsheets. It is worth noting that similar XRD patterns of the nanoplates and nanosheets samples have also been observed from highly crystallized silver and gold nanoplates, where the XRD patterns of the products only show the {111} facets family, such as the (111) and (222), while the other planes, such as (200) and (311), could be omitted.1,14 Figure 3 shows typical SEM images and an EDX pattern of the as-synthesized nanoplates. Figure 3a,b and Figure S1 (see Supporting Information) show that the products contain a large quantity of platelike materials. The histograms shown reveal that the in-plane sizes of the products are between 200 and 500 nm, and their thicknesses are between 15 and 25 nm. Figure 3e,f shows the high magnification SEM image of the assynthesized nanoplates indicating that the size and morphology of the product is quite uniform. Figure 3g shows one typical nanoplate with its basal plane parallel to the silicon substrate, while Figure 3h shows one typical nanoplate with its basal plane upright to the silicon substrate, which revealed that the nanoplate has a thickness of 20 nm. Figure 3i is an EDX spectrum obtained from a single nanoplate shown in the inset of Figure 3g. Only

Figure 6. (a) Low and (b, c) high magnification SEM images of the as-synthesized BiOCl thick microsheets; (d) typical edge images of the thick microsheets; (e-g) typical TEM images of trigonal, tetragonal, and multiangular thick microsheets, respectively; and (h) the corresponding SAED pattern of the thick microsheet in panel e.

BiOX (X ) Cl, Br) Micro- and Nanostructures

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Figure 7. TEM images (a), SEM image (b), and the corresponding SAED patterns (c) of the as-synthesized BiOBr nanoplates; TEM images (d, e) and the corresponding SAED patterns (f) of the as-synthesized BiOBr nanosheets; SEM image (g, inset j), TEM image (h), and the corresponding EDX pattern (j) of the as-synthesized microsheets; and powder XRD patterns (i) of the BiOBr nanoplates (inset a), nanosheets (inset b), microsheets (inset c), and the reference pattern (inset vertical bars) of the JCPDS cards (Card No. 85-0862).

Bi, O, and Cl peaks are observed in this spectrum (silicon signal from the silicon substrate), suggesting that the nanoplates are composed of mainly Bi, O, and Cl. Quantitative EDX analysis shows that the atom ratio of Bi/O/Cl is 1:0.986:0.973, close to 1:1:1, indicating that the composition of the as-synthesized products is BiOCl. The morphologies of the as-synthesized nanoplates were further investigated by TEM as shown in Figure 4a,b, which also shows the formation of nanoplates. The microstructure of an individual BiOCl nanoplate was further investigated in detail by HRTRM and SAED. A typical HRTEM image of a single BiOCl nanoplate is shown in Figure 4c,d. The lattice spacings are both about 0.285 and 0.285 nm and correspond to (110) and (110) plane spacing of tetragonal phase BiOCl (JCPDS Card No. 85-0861). The SAED patterns show in Figure 4e taken along the [001] zone axis is a spot pattern, which reveals that the nanoplate is single crystalline in nature. In addition, the bright spots shown in the SAED patterns could be indexed to (110), (110), and (200) of tetragonal phase BiOCl (JCPDS Card No.

85-0861). Figure 4 also reveals that the BiOCl nanoplates are well-crystallized, free from dislocation and stacking faults. Single crystal as-synthesized BiOCl nanosheets were obtained with higher precursor concentrations at higher temperatures as compared to the nanoplates. The XRD and the EDX patterns of the BiOCl nanosheets were similar to those of the BiOCl nanoplates. The SEM and TEM images of Figure 5a-d clearly showed the formation of nanosheets. Figure S2 of the Supporting Information shows the identification of a single nanosheet with a thickness of ∼60 nm, and the as-synthesized BiOCl nanosheets were typically 2-50 µm in in-plane size and 40-80 nm in thickness. A typical HRTEM image of a single BiOCl nanosheet is shown in Figure 5e. The lattice spacings are both about 0.285 and 0.285 nm and also correspond to (110) and (110) planes spacing of tetragonal phase BiOCl (JCPDS Card No. 85-0861). The typical SAED patterns shown in Figure 5e and inset Figure 5d taken along the [001] zone axis is a spot pattern, which reveals that the BiOCl nanosheets are single crystal in nature. In addition, the bright spots shown in the SAED patterns could

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Figure 8. TEM images and the corresponding SAED pattern of the as-obtained BiOCl nanostructures collected at different reaction stages: (a) suspension before the hydrothermal treatment, (b) hydrothermal treatment for 2 h, and (c) hydrothermal treatment for 10 h.

be indexed to (110), (110), and (200) of tetragonal phase BiOCl (JCPDS Card No. 85-0861). Figure 5 also reveals that the BiOCl nanosheets are well-crystallized, free from dislocation and stacking faults. Single crystal as-synthesized BiOCl thick microsheets were obtained with higher precursor concentrations at higher temperatures as compared to the nanosheets. The XRD and the EDX patterns of the thick microsheets were similar to those of the BiOCl nanosheets. The SEM images shown in Figure 6a-d clearly showed the formation of thick microsheets. The assynthesized thick microsheets were usually 10-100 µm in inplane size and 200-1000 nm in thickness. Figure 6d shows a typical microsheet with a thickness of 400 nm. The typical TEM images in Figure 6e-g show the typical trigonal, tetragonal, and multiangular thick microsheets, respectively. The SAED patterns show in Figure 6h taken along the [001] zone axis are spot patterns, which also reveal that the nanoplate is single crystal in nature. In addition, the bright spots shown in the SAED patterns could be indexed to (110), (110), and (200) of tetragonal phase BiOCl (JCPDS Card No. 85-0861). Under a similar synthesis strategy, 2D BiOBr micro- and nanostructures can also be synthesized. Figure 7 shows TEM images, SEM images, SAED patterns, and EDX patterns of the as-synthesized BiOBr nanoplates, nanosheets, and microsheets. The SAED patterns shown in Figure 7c,f taken along the [001] zone axis is a spot pattern, which reveals that the nanoplate is single crystal in nature and with the tetragonal phase. In addition, the bright spots shown in the SAED patterns could be indexed to (110), (110), and (200) of tetragonal phase BiOBr (JCPDS Card No. 85-0862). Figure 7i shows the typical XRD patterns of the as-synthesized 2D BiOBr nanostructures, such as nanoplates, nanosheets, and microsheets. The experimental XRD profiles taken from the as-synthesized BiOBr samples are very

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similar to BiOCl samples, showing that all of the peaks may be indexed as the tetragonal phase BiOBr (cell constants a ) 3.920 Å, c ) 8.110 Å; JCPDS Card No. 85-0862). No peaks of metal Bi or any other phases were detected, indicating that the products are very high-purity, single-phase samples. Figure 7j is an EDX spectrum obtained from a single nanoplate shown in the Figure 7j inset. Only Bi, O, and Br peaks are observed in this spectrum (silicon signal from the silicon substrate), suggesting that the nanoplates are composed of mainly Bi, O, and Br. Quantitative EDX analysis shows that the atom ratio of Bi/O/Br is 1:0.975:0.968, close to 1:1:1, indicating that the composition of the as-synthesized products is BiOBr. The growth mechanism of the 2D single-crystalline BiOX (X ) Cl, Br) nanostructures was also investigated. As shown in Figure 1, the sizes of the irregular shaped Bi particles range from 50 to 200 µm. Similar to metal bulk antimony, it is known that the oxidation of metal Bi by H2O2 in water is very slow, when Bi is oxidized in a humid environment.10,31 It is believe that, in our experiments, halide ions (Cl- and Br-) possibly acted as a catalyst; thus, the presence of halide ions would change the microenvironment for the electrochemical reaction; therefore, the spontaneous oxidation reaction of Bi metal is accelerated drastically and transforms into Bi oxhydrohalides nanoparticles, while oxygen is reduced in the reaction. Under further hydrothermal treatment, the Bi oxhydrohalides nanoparticles could turn out to be 2D Bi oxyhalides nanostructures. Figure 8 shows the TEM images and the corresponding SAED pattern of the as-obtained products collected at different reaction stages. The TEM image shown in Figure 8a is the product before the hydrothermal treatment, revealing the formation of spherical Bi oxhydrochlorides [BiCl(OH)] nanoparticles with a size between 5 and 20 nm. The corresponding SAED ring pattern shown in Figure 8b indicated that the Bi oxhydrochlorides are multicrystalline in nature. The XRD pattern of the same sample shown in Figure S3 of the Supporting Information also shows that the Bi oxhydrochlorides product is in a multicrystalline structure, and the peak broadening in the XRD patterns clearly indicates that the product is a very small size, consistent with the TEM observations. Unfortunately, this XRD pattern is not indexed in the literature or JCPDS card up to now, and further investigation is still in progress. Figure 8c shows the product of hydrothermal treatment at 160 °C for 2 h; the corresponding SAED revealed a spot-built ring pattern as shown in Figure 7d, indicating that the BiOCl products have better crystallinity after hydrothermal treatment, and the pattern corresponds to tetragonal phase BiOCl. Figure 8e shows the product of hydrothermal treatment at 160 °C for 10 h; a well-distinguished spot pattern shown in Figure 8f indicated that the BiOCl products after long time hydrothermal treatment are single-crystalline in nature; the pattern also corresponds to tetragonal phase BiOCl. These formation process investigations revealed that that the BiOCl nanoparticles could side-by-side self-assemble to form nanoplates by increasing the in-plane size. To further investigate the formation process from the Bi oxhydrochlorides nanoparticles before hydrothermal treatment to the 2D Bi oxychlorides nanostructures after hydrothermal treatment, TGA and FTIR spectra were performed for these two different samples. As shown from TGA curves in Supporting Information Figure S4, the first weight loss in the range of 30 to ∼300 °C is due to the release of free water; the second weight loss in the range of ∼300 to ∼500 °C is attributed to the decomposition of oxhydrochlorides, if Bi oxhydrochlorides existed, while the third weight loss in the range of ∼500 and 900 °C is attributed to the decomposition of Bi oxychlorides.

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Figure 9. (a, b) SEM images of the as-synthesized BiOCl microsheets after hydrothermal treatment at 200 for 5 h and (c) schematic drawing of the BiOX (X ) Cl, Br) Bi oxyhalides crystal. (d) Low (upper left) and high magnification TEM images and SAED pattern (upper right) of the as-synthesized BiOCl products. The HRTEM image is an enlarged area of the marked area of “1” in the upper left. The arrows indicate the layer-by-layer piling up of thin nanosheets to form thick microsheets.

For the sample before the hydrothermal treatment, the TGA curve clearly indicated that the weight loss between 295.25 and 506.72 °C is about 20.6%, indicating that the sample is Bi oxhydrochlorides, while for the sample after hydrothermal treatment, the weight loss of between 298.46 and 504.37 °C is about 1.4%, indicating that the product after hydrothermal is Bi oxychlorides. Furthermore, as shown in the FITR spectra of Figure S5 of the Supporting Information, for the sample before the hydrothermal treatment, one should see an obvious and broad peak around 3420 cm-1, which is attributed to the H-O stretching vibrations, and revealed that the product is Bi oxhydrochlorides in nature. However, for the sample after hydrothermal treatment, there are no noticeable peaks around 3420 cm-1 that appeared, and the missing of the H-O stretching vibrations further confirmed that the sample is Bi oxychlorides in nature. It is found that the presence of PVP has a significant influence on the formation of Bi oxyhalides nanoparticles. PVP is present as the surface stabilizer, and its long polymeric chain structure will completely surround one or more nuclear Bi oxyhalides.10 What’s more, it is found that PVP could selectively adsorb on the specific plane of Bi oxyhalides and control the growth of the 2D nanostructures. As shown in Supporting Information Figure S6, comparative experiments were performed and took the recipe of synthesis of BiOCl nanosheets for an example, where the amount of PVP varied from 100, 400, to 1600 mg, while keeping all of the other parameters as the same. The SEM images show that with a low PVP concentration, the product revealed the morphology of giant aggregates of the nanosheets, while at a high PVP concentration, the product revealed the morphology of individual small spherical submicroparticles with

the diameter between 200 and 500 nm. On the basis of the above discussion, the reaction that occurred in the formation of the Bi oxyhalides crystal nucleus may be illustrated as follows: PVP

2Bi + 3H2O2 + 2NaX 98 2BiOX + 2NaOH + 2H2O H2O

(1) where X ) Cl, Br. Hydrothermal treatment is a common method to synthesize zeolite and molecular sieve crystals.7 This method exploits the solubility of almost all inorganic substances in water at elevated temperatures and pressures and subsequent crystallization of the dissolved material from the fluid.7 In recent years, hydrothermal synthesis has been demonstrated to be an effective and powerful technique for the growth of nanostructured materials.32,33 In our experiments, the hydrothermal treatment will enable us to transform Bi oxyhalide nanoparticles into well-defined 2D single-crystalline nanoplates and nanosheets. It was found that the in-plane size and the thickness of the product would both increase at the higher temperature and high precursor concentration (see Scheme 1). Figure 9 shows the investigation of BiOCl microsheets after hydrothermal treatment at 200 for 5 h. The SEM images in Figure 9a,b reveal that through layer-by-layer piling up of thin nanosheets could result in the formation of thick microsheets as indicated by the arrows. The TEM and HRTEM images and the SAED pattern shown in Figure 9d further indicate the piling up process in the formation of the thick nanosheets. In addition, it is revealed that the product is also single crystal in nature during the piling up process. Figure 9c shows the schematic

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absorption edges at about 360, 370, 500, and 560 nm in the near-UV and visible light regions, respectively. It is found that the UV-vis diffuse reflectance edges of the BiOCl and BiOBr samples have a clear blue shift by substitute Br with Cl. For the nanoplates and nanosheets samples of the same material, it is found that the UV-vis diffuse reflectance edges also blue shift by a decrease in the thickness from 40-80 nm to about 15-25 nm, which is possibly due to the size-dependent effects.7 However, as shown in Figure S7 of the Supporting Information, the UV-vis diffuse reflectance spectra of the nanosheets and microsheets samples are very close due to their large sizes, and no obvious shift is observed due to their large sizes (over 40 nm). As a crystalline semiconductor of indirect transition, the optical absorption near the band edge follows the formula h ) A(h - Eg)2, where Eg and A are the absorption coefficients, light frequency, band gap energy, and a constant, respectively. The band gap energies (Eg) of BiOX can be thus estimated from a plot of the (RhV)1/2 vs photon energy (hV) plots as show in Figure 10b. The estimated band gap energies of the resulting samples were about 3.5, 3.3, 2.3, and 2.1 eV for BiOCl nanoplates, BiOCl micro- and nanosheets, BiOBr nanoplates, BiOBr microand nanosheets, respectively, which are close to the values reported in the literature.23,28,30,34,35 Conclusions

Figure 10. UV-vis diffuse reflectance spectra (a) and the (RhV)1/2 vs photon energy (hV) plots (b) of the as-synthesized 2D BiOX microand nanostructures: (1) BiOCl nanoplates, (2) BiOCl micro- and nanosheets, (3) BiOBr nanoplates, and (4) BiOBr micro- and nanosheets.

drawing of the BiOX (X ) Cl, Br) crystal.21,27 Bi oxybromides and oxychlorides have a tetragonal PbFCl- type structure with a space group of P4/nmm. This is known to be a layered structure, which is constructed by the combination of the halides (Cl- or Br-) ion layer and the metal-oxygen (Bi-O) layer. In our opinion, the layered structure of Bi oxyhalides suggests that nanoplates and nanosheets may be formed under appropriate conditions. It is assumed that the microsheets produce growth through a typical dissolution-recrystallization process followed by Ostwald ripening mechanism, in which the growth progress is controlled by mass transport and by surface equilibrium of addition and removal of individual monomers.4,33 The room temperature band gap (Eg,ind ∼ 3.455 eV, Eg,dir ∼ 3.50 eV) of the bulk Bi oxychlorides crystal was determined by the optical absorption spectrum in the 1970s.34 Recently, several groups have reported the optical bandgap of Bi oxyhalides nanomaterials. For example, Zhang et al. reported that the band gaps of the BiOCl and BiOBr hierarchical nanoplate microspheres were about 3.22 and 2.64 eV, respectively.30 Zheng et al. reported that the layered compound BiOCl has an optical indirect band gap of 3.46 eV.23 Very recently, Tang et al. reported that the band gap of the obtained BiOBr microspherical architectures was estimated to be 2.54 eV.35 Herein, as shown in UV-vis diffuse reflectance spectra of the BiOX samples in Figure 10a and Figure S7 of the Supporting Information, the BiOCl and BiOBr samples have intense

In summary, a facile wet chemistry approach to large-scale synthesis of 2D single-crystalline BiOX (X ) Cl, Br) nanoplates, nanosheets, and microsheets by a simple wet chemistry approach of H2O2 direct oxidation of bulk metal Bi particles followed by a hydrothermal treatment is demonstrated for the first time. The in-plane size and thickness of nanoplates, nanosheets, and microsheets can be conveniently tailored by varying the temperature and the concentrations of the Bi precursor, which may be of certain generality in controllable synthesis of other 2D nanostructures. The formation process investigation revealed that under hydrothermal treatment the spherical Bi oxhydrohalides nanoparticles could be side-by-side self-assembled to form Bi oxyhalide nanoplates by increasing the in-plane size, and nanosheets could be piled up to form Bi oxyhalide thick microsheets by increasing the thickness. The UV-vis diffuse reflectance spectra indentified the estimated band gap energies of the 2D single-crystalline micro- and nanostructures. It is expected that the present study could be extended to facile, largescale synthesis of various multicomponent 2D inorganic microand nanostructures, which would have better performances than the corresponding spherical nanoparticles and would be the new members in the family of advanced functional inorganic materials well-applied in industry. By the suitable choice of source and synthetic parameters, it is reasonable to expect that the present study could be extended to other multicomponent 2D nanostructured functional materials. Acknowledgment. The current investigations were financially supported by the National Natural Science Foundation of China (Nos. 60736001, 60572031, and 20571080). Supporting Information Available: Enlarged SEM images of BiOCl nanoplates, additional SEM images of BiOCl nanosheets, XRD pattern of the as-synthesized suspension before the hydrothermal treatment, TGA curves and FTIR spectra of the suspension before the hydrothermal treatment and the microsheets after hydrothermal treatment, SEM images of the BiOCl products obtained with different amounts of PVP, and UV-vis diffuse reflectance spectra of the BiOCl nanosheets and BiOCl microsheets. This material is available free of charge via the Internet at http://pubs.acs.org.

BiOX (X ) Cl, Br) Micro- and Nanostructures

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