Article pubs.acs.org/crystal
Ligand-Free One-Step Synthesis of {001} Faceted Semiconducting BiOCl Single Crystals and Their Photocatalytic Activity Anupam Biswas,†,∥ Raja Das,†,‡,∥ Chandan Dey,†,‡ Rahul Banerjee,†,‡ and Pankaj Poddar*,†,‡,§ †
Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110 001, India § Centre of Excellence on Surface Science, CSIR - National Chemical Laboratory, Pune 411008, India ‡
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S Supporting Information *
ABSTRACT: Herein, we report one-step ligand-free hydrothermal synthesis of predominantly {001}-faceted micron-sized single crystals of bismuth oxychloride (BiOCl). The structural chlorine is obtained by in situ generation of chloride ions. From Raman spectra, we could resolve the Eg transition. Photocatalytic activity of as-synthesized crystals showed 78% degradation of RhB dye under a xenon source after 120 min of exposure.
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were mixed together in 35 mL of distilled water and then transferred to a 70 mL Teflon-lined autoclave (material details are given in the Supporting Information). The solution was stirred for 1 h, and subsequently, 5 mL of nitric acid (69−71%) was added. The autoclave was heated at 200 °C for 4 days and then allowed to cool naturally. We found that colorless, shiny, square-shaped single crystals were formed in the reaction vessel (Figure S8, Supporting Information). Powder XRD (PXRD) and single-crystal XRD data confirmed that the asformed crystals are in the tetragonal BiOCl phase (Figure S6, Supporting Information). To investigate the role of nitric acid, we performed the reaction by varying the volume of concentrated nitric acid. With the increase in acid volume from 1.75 to 7 mL, the size of the square-shaped crystals increased (Figure S8, Supporting Information). The possible cause of the increase in crystal size with increase in volume of nitric acid is not yet understood. It was found that the solution color also changed from colorless to pale yellow. The increase in acid volume may cause reversal of chloride ion transfer from structure to solution, which results in the yellow color of the solution. To further investigate the role of nitric acid, the reaction was carried out without adding nitric acid; but to our surprise, we did not observe any crystal formation. We found the formation of a red-colored precipitate. Here in NaBiO3, Bi exists in the +5 oxidation state, and in acidic medium, it goes into the +3 oxidation state. The presence of nitric acid makes chloride ions available from FeCl2·4H2O. To confirm the source of chloride ions in bismuth oxychloride, a similar reaction was also performed in hydrochloric acid medium, but it was found that there was no formation of precipitate and we ended-up with a clear yellow solution. Therefore, in this reaction, chloride ions might have generated in situ after addition of nitric acid, which facilitates the release of chloride ions from iron chloride in the acidic medium. The reaction scheme of BiOCl single crystal formation is shown in Figure 1a.
INTRODUCTION In inorganic chemistry, preparation of bulk or nanosized single crystals of materials has potential applications. However, the synthesis of bulk or nanosized crystals with proper control over crystallinity is always a challenge.1−7 The BiOCl is one of the important multicomponent V−VI−VII semiconductor that draws considerable attention due to its photocatalytic properties.7−11 The crystal structure was solved, which defines that BiOCl consists of a tetragonal layer with Cl-Bi-O-Bi-Cl sheets stacking with lattice constants a = b = 3.887 Å and c = 7.354 Å.12 Recently, it has been shown that the {001} facet is relatively reactive and thus shows enhanced photocatalytic performance.13−15 There is no theoretical study for BiOCl to prove the above fact. To understand photocatalytic property, it is important to synthesize purely single-crystalline BiOCl with fully exposed {001} facets. Different types of BiOCl nanostructures, such as nanosheets,10,13 nanoflowers,16 nanospheres,7 nanoplates,8 and nanowires,17 were prepared by various methods. As we know that physical properties can be tuned by the properly oriented growth of single crystals, the reactivity and selectivity can be tuned with the proper understanding of the intrinsic geometric structure and crystallographic orientation.18 Earlier, BiOCl bulk single crystals were prepared by vapor transport and solid-state methods.19,12 However, the synthesis of BiOCl single crystals in solution phase has not been achieved so far. This is for the first time, we report the chemical synthesis of micrometer-sized single crystals of bismuth oxychloride (BiOCl). The uniqueness of the synthesis lies in the fact that it is ligand-free and a single-pot method with maximum exposed {001} facets.
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EXPERIMENTAL SECTION
Received: September 26, 2013 Revised: November 27, 2013 Published: December 10, 2013
In a typical hydrothermal synthesis,1 mmol of sodium bismuthate (NaBiO3) and 1 mmol of iron chloride tetrahydrate (FeCl2·4H2O) © 2013 American Chemical Society
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with space group P4/nmm. It can be concluded from the observed PXRD spectrum that the {001} facet is responsible for the formation of square-shaped crystals (Figure S8, Supporting Information), directed along the ab plane. No other impurity phases, such as Bi2O3 and BiCl3, were found in the PXRD, which clearly emphasizes that the obtained product is highly pure and single phase. Thermal gravimetric analysis (TGA) showed that crystals were stable up to 600 °C (Figure S7, Supporting Information). The selected area electron diffraction (SAED) pattern of BiOCl crystals shows a sharp spot pattern, which was expected for the single crystalline sample (Figure S9, Supporting Information). The diffraction spots were assigned according to JCPDS 06-0249. To understand the photocatalytic behavior, the optical band gap of BiOCl was estimated from absorption spectra (Figure S10, Supporting Information). The calculated value of the band gap was ∼2.9 eV for the square-shaped BiOCl single crystals and is in good agreement with Xiong et al.8 It was reported earlier that the square-shaped BiOCl crystal has a narrow band gap that is different from its theoretical value (3.46 eV).19 In bulk crystals, the optical band gap is anisotropic and depends on the direction of the crystal. A photoluminescence (PL) spectrum was used to investigate the efficiency of charge carrier trapping, migration, and transfer. The sample was excited at a wavelength of 320 nm, and the emission peak was recorded in the range of 340−420 nm (Figure S11, Supporting Information). The low PL intensity is due to the high electron−hole recombination rate and consequently decreases the quantum efficiency7 The optical mode of Raman spectra deduced by Fateley et. al.20 and Cao et. al.21 is represented as
Figure 1. (a) Schematic representation of BiOCl synthesis. (b) Building block of BiOCl crystal. (c) View of BiOCl crystal along crystallographic c axis corresponds to the 001 plane. (d) View of BiOCl crystal structure along crystallographic a axis showing two parallel planes. (e) Space-filling model of BiOCl single crystal (violet: Bi; green: Cl; red: O).
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RESULTS AND DISCUSSION In the crystal structure of BiOCl (space group P4/nmm), the “Bi” is connected with four oxygen atoms and four chlorine atoms at either sides (Figure 1b). Each “Bi” is connected to four Bi via μ4-O and four Bi via μ4-Cl (Figure 1c). There is no such connectivity along the c direction, which results in a 2-D lamellar sheetlike structure in the ab plane (Figure 1d). There are two types of “Bi−O−Bi” angles, 114.22° and 107.15°, where the two Bi ions were connected to Cl ions and the other type is formed only by a Bi−O network, respectively (Figure S2, Supporting Information). The structure consists of two types of bonds, i.e., Bi−Cl and Bi−O, where bond distances were 3.055 and 2.314 Å, respectively. In the crystal structure, the Bi−Cl−Bi, O−Bi−Cl, and O−Bi−O angles are 79.09, 72.97, and 72.85°, respectively. The interlayer distance between two parallel lamellar-like sheets is 3.5 Å. All the PXRD peaks match with the single crystal generated pattern (Figure 2).The peak corresponding to the {001} facet has higher intensity, so it can be concluded that the {001} facet grows predominantly over others. The PXRD pattern was indexed with tetragonal BiOCl (JCPDS: 06-0249) (Figure S6, Supporting Information)
Γ = 2A1g + 2A 2u + B1g + 3Eg + 2E u
where g modes are Raman-active and u modes are infraredactive.22 Room-temperature Raman spectra (Figure 3) show
Figure 3. Raman spectra of BiOCl crystals. Inset shows the zoom view from 130 to 160 cm−1 that resolves the Eg external Bi−Cl transition at 149 cm−1 by fitting.
three strong bands at 61, 145, and 201 cm−1 along with one weak band at 398 cm−1. The intense Raman band at 61 cm−1 comes from the Eg Bi−O−Cl stretching mode, whereas the 145 cm−1 band is assigned to the A1g Bi−Cl stretching mode.7 It was discussed in literature that the Eg external Bi−Cl stretching mode is always masked by the A1g Bi−Cl stretching mode.20 However, when we took a closer look at the peak around 145 cm−1, we found it to be asymmetric. By fitting the peak, we were able to resolve the Eg (145 cm−1) and A1g (149 cm−1) transitions. The band at 201 cm−1 can be assigned to Eg internal Bi−Cl stretching. The weak band at 398 cm−1 is due to the Eg and B1g modes brought by the motion of the oxygen atom. The
Figure 2. Comparison of the experimental PXRD pattern of assynthesized BiOCl crystals (top) with the one simulated from its single crystal structure (bottom). Experimental PXRD pattern of assynthesized BiOCl crystals shows oriented growth along the {001} facet. 237
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shows the absence of pores in the crystals. From the result, it can be concluded that the photocatalytic activity of BiOCl not only depends on exposed facets but also depends on other parameters, such as internal voids, pore size, and exposed surface to the dye molecule. However, to increase the photocatalytic activity by increasing the surface area, one makes a compromise on the favored growth of active facets. Therefore, there is a trade-off between synthesizing the materials with a large surface area by creating more pores in the material and creating material with desired facets. In bulk single crystals, because of low surface area, there is less interaction with the dye, which results in lower photocatalytic activity. Our as-synthesized crystals are shiny; therefore, it is expected that the surface is optically smooth, which was also responsible for the lowering in photocatalytic activity. To check the stability of crystals after photocatalytic degradation of RhB, we have done PXRD of crystals. The PXRD pattern shows no change in the tetragonal phase of BiOCl crystals after photocatalytic degradation of RhB. To our surprise, the PXRD pattern of BiOCl crystals after photocatalytic degradation of RhB shows more prominently {001} exposed facets than the as-obtained samples (Figure S16, Supporting Information). To rule out any orientation effect in PXRD, we carried out multiple measurements and found that the intensity of the {001} facet increased with diminished (101), (102), (212), and (220) planes of BiOCl crystals after photocatalytic degradation (Table S2, Supporting Information). This probably indicates that the surface roughness of the crystals after the photodegradation has been reduced, and we need more insights to understand the phenomenon.
shift of the peak position is due to highly oriented growth of the single crystal. The photocatalytic activity of the {001} facet oriented BiOCl was checked by the degradation of rhodamine blue dye (RhB). A 5 mL portion of 10−5 M RhB was taken along with 5 mg of prepared sample in a bottle and stirred in the dark for 3 h to attain the adsorption−desorption equilibrium. To check the photocatalytic activity, the solution was irradiated by a 400 W xenon lamp source for 120 min and UV−vis absorption spectra of RhB were recorded after varying times of xenon lamp exposure (Figure 4). From the time-dependent UV−vis spectra,
Figure 4. Photocatalytic degradation of RhB at different times of exposure to 400 W xenon lamp source. Inset: photocatalytic degradation of RhB (maximum absorbance) with different times of exposure to 400 W xenon lamp source where, after 120 min, 78% of RhB was degraded.
it was found that, after 120 min, 78% of the RhB was degraded. The plot of absorption maxima versus time shows that the degradation was almost a linear function of the exposure time and completes at about 150 min (Figure 4, inset). To check the sustainability of our as-synthesized crystals, we have performed the photocatalyst experiment up to four cycles. In the second, third, and fourth cycles, our as-synthesized crystals are able to degrade 62% of RhB dye after 120 min of exposure (Figure 5),
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CONCLUSION To summarize, for the first time, we report a successful ligandfree chemical synthesis of BiOCl micrometer-sized single crystals by a one-step hydrothermal method. PXRD data revealed that as-obtained crystals were predominantly {001}faceted. Raman spectra could resolve the Eg (149 cm−1) transition that was not reported in the literature so far. Photocatalytic degradation of RhB showed that the photocatalytic activity of BiOCl does not depend only on {001} exposed facets but also depends on other parameters.
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ASSOCIATED CONTENT
S Supporting Information *
Crystallographic data (CIF) CCDC 922288 and additional supporting data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 5. Photocatalytic degradation of RhB dye up to four cycles.
Author Contributions ∥
which shows reproducibility of catalytic behavior. It was observed that there is a shift of absorption peak toward the lower wavelength (hypsochromic shift) due to step-by-step degradation of RhB. As we discussed earlier, the {001} facet has higher photocatalytic activity.13−15 In the as-synthesized sample, the {001} facets were most exposed, but despite this, we did not get reasonably high photocatalytic activity. We checked the surface area of the BiOCl crystals using BET; it shows a surface area of 14 m2/g, which might have come from surface adsorption (Figure S17, Supporting Information). This
A.B and R.D. contributed equally to the work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS P.P. acknowledges the Centre for Excellence in Surface Science at the National Chemical Laboratory and network project Nano-SHE funded by the Council of Scientific and Industrial Research (CSIR), India, and the Department of Science & Technology (DST), India (DST/INT/ISR/P-8/2011). A.B. 238
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acknowledges the Department of Science & Technology (DST), India (DST/INT/ISR/P-8/2011), for financial support. R.D. and C.D. acknowledge CSIR, India, for financial support.
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