Mass Production and Photocatalytic Activity of Highly Crystalline

Oct 27, 2010 - dispersive spectrum analysis (EDS), X-ray diffraction (XRD), N2 ... The Bi-Ti-O system have drawn great interest due to their physical ...
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Environ. Sci. Technol. 2010, 44, 8698–8703

Mass Production and Photocatalytic Activity of Highly Crystalline Metastable Single-Phase Bi20TiO32 Nanosheets TENGFEI ZHOU AND JUNCHENG HU* Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, South-Central University for Nationalities, Wuhan, 430074, People’s Republic of China

Received June 11, 2010. Revised manuscript received September 19, 2010. Accepted October 12, 2010.

Highly crystalline metastable bismuth titanate (Bi20TiO32) nanosheets are prepared via a simple green wet chemical route for the first time. The Bi20TiO32 photocatalysts were characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy dispersive spectrum analysis (EDS), X-ray diffraction (XRD), N2 adsorption-desorption (BET), and UV-vis diffuse reflectance spectroscopy (DRS). Inspiringly, Bi20TiO32 nanosheets showed high photocatalytic activity for the degradation of nonbiodegradable azo dye under simulated sunlight and visible-light irradiation. The experimental results showed that the photocatalytic activity of the Bi20TiO32 nanosheets was superior to the commercial Degussa P25 TiO2, and demonstrated that the morphology and crystal structure have a distinct effect on the photocatalytic activity. The reasons for the high photocatalytic activity and the formation mechanism of Bi20TiO32 nanosheets are also discussed.

Introduction Environmental and energy issues are very important topics on a global scale. Natural energy, such as sunlight, can be employed to help curb the damage that polluted wastewater has on the environment. Semiconductor photocatalysis offers the potential technology for complete elimination of toxic chemicals through its efficiency and potentially broad applicability (1, 2). Various new compounds and materials for photocatalysis have been synthesized in the past few decades. A successful example is TiO2, a metal oxide often used as a catalyst in photochemistry. However, fast recombination rate and poor solar efficiency (maximum 5%) have hindered the practical applications of most photocatalysts (3, 4). Therefore, studies on attempting to eliminate these drawbacks on photocatalysts have been performed (5). The surface structure of photocatalysts plays an important role on their photocatalysis because the photocatalytic reaction or photoelectron conversion takes place only when photoinduced electrons and holes are available on the surface. Synthesis of nanoscale semiconductor photocatalysts becomes more attractive because of their different physical and chemical properties from bulk materials. Meanwhile, most studies report only micro to millimolar amounts of * Corresponding author phone: 86 27 67841302; e-mail: [email protected]. 8698

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photocatalysts with intrinsically high cost. Therefore, the development of more economically viable for the large-scale synthesis of nanoscale photocatalysts with high catalytic activity and easy separation is a challenge for researchers (6, 7). The story of C60 demonstrates the importance of thermodynamically metastable substances that have applications in many fields. So researchers seek to understand how to create new substances. One strategy is to explore materials with thermodynamic metastability, that is, to search for materials seemingly unfavored by their higher energy, yet persist due to barriers that impede conversion to lower energy forms (8). Bismuth titanates belong to a complicated system that includes several different phases such as Bi2Ti2O7, Bi2Ti4O11, Bi4Ti3O12, Bi12TiO20, and Bi20TiO32. The Bi-Ti-O system have drawn great interest due to their physical properties and technological applications (9-11). For example, Murugesan et al. reported that Bi2Ti2O7 nanorods exhibit photocatalytic activity toward hydrogen generation and degradation of a textile dye under UV-visible light (12). Zhou et al. demonstrated the potential of Bi12TiO20, as a visible-light photocatalyst for the oxidation of methanol to CO2 (13). Mesoporous bismuth titanates photodegradation of DCP under visible-light irradiation has been reported in Kong’s communication (14). Because different bismuth titanate phases are formed depending on different chemical compositions and processing conditions, so highly crystalline and single phase bismuth titanate are difficult to obtain. Metastable Bi20TiO32 is a photo active member of the bismuth titanates family that has the potential to possess the desired qualities of a photocatalyst. Quenching method is the most common synthesis route for this material. Due to high temperatures, this route typically results in an irregular morphology and large agglomerated particles as well as a low surface area (15). And the properties of nanomaterials depend not only on their composition but also on their structure, morphology, phase, shape, size, distribution, and spatial arrangement (16-19). The controlling over morphology and size of nanostructures has become a fundamental issue in the design and synthesis of nanomaterials. It has been reported that a strong electrostatic interaction between inorganic crystal faces and organic species results in morphological evolution. Along with fundamental studies, we believe that the next challenge will be the new chemical routes for the preparation of nanostructured and functional nanomaterials. Meanwhile, the wet chemical synthesis and morphological control of metal-oxide nanomaterials have not been fully demonstrated in recent studies, especially in terms of two-dimensional (2D) nanostructures. An improved strategy for the simultaneous controlling of chemical reaction, oxidation state, crystal phase, and morphology is required for the next stage of materials chemistry. Therefore, the development of a synthesis of metastable two-dimensional (2D) nanostructures that is cost-effective, with mild conditions, suitable for large-scale production, with high catalytic activity, and easy separation represents a critical challenge to the practical application of these nanomaterials. Herein, in our study, for the first time, highly crystalline metastable phase bismuth titanate (Bi20TiO32) nanosheets, produced on a gram-scale, were synthesized on the basis of our previous work (20-22). The synthesis of metastable Bi20TiO32 phase on a gram-scale could not be fabricated by the traditional method. So wet-chemical methods were used in our study since they can be controlled from the molecular precursor to the final product to give highly pure and homogeneous materials. These methods allow for lower 10.1021/es1019959

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reaction temperatures control over the size and morphology of the particles, and preparation of metastable phases (23). The morphology and size of the formed Bi20TiO32 nanosheets can be easily tuned by varying the experimental parameters. Its photocatalytic activity for degradation of dyes in aqueous solution under simulated sunlight or visible-light irradiation has been evaluated.

Experimental Section Materials and Reagents. Bismuth nitrate (Bi(NO3)3 · 5H2O) was obtained from Tianjin Kermel Chemical Reagents Development Centre (Tianjin, China). Methanol, ethanol, anhydrous acetic acid, benzyl alcohol, poly(vinyl alcohol) (PVA), potassium hydroxide, brilliant red X3B, and urea were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Titanium isopropylate was purchased from Alfa Aesar. All chemicals were analytical grade reagents. Degussa P25 (∼80% anatase and ∼20% rutile) was purchased from Degussa (China) Co., Ltd. Deionized and doubly distilled water was used in this work. Sample Preparation. In a typical synthesis procedure, 9.70 g Bi(NO3)3 · 5H2O was dissolved in 60 mL anhydrous acetic acid. After the Bi(NO3)3 · 5H2O dissolved completely, 4.26 g Titanium isopropylate was slowly added under vigorous magnetic stirring at room temperature. Then 200 mL methanol was injected under stirring until the white gelatinous fluid became transparent. Urea and 7.8 g benzyl alcohol was added to the mixture in the end. After stirring for 1 h, the solution was transferred into an autoclave and the reaction mixture was purged with 1 MPa N2 3×, and then a pressure of 1 MPa N2 was imposed before initiating heating, then heated to 180 °C, maintained for 2 h, then to 240 °C, and then held for an additional 2 h; finally, the vapor inside was vented. After the supercritical fluid drying (SCFD), a black powder was collected, rinsed several times with absolute ethanol and distilled water and subsequently calcined in air with a ramp rate of 3 °C/min to 300 °C, then maintained at 300 °C for 5 h. Bismuth titanate nanowires with the growth direction is [010] were synthesized and used for comparison (see Supporting Information for the detailed synthetic procedure) (24). Characterization. The crystalline structure of the catalysts were characterized by power X-ray diffraction (XRD) employing a scanning rate of 0.05°/s in a 2θ range from 10° to 80°, in a Bruker D8. Advance using monochromatized Cu KR radiation. The morphologies and sizes of the samples were observed by transmission electron microscopy (TEM), which were taken on a Tecnai G20 (FEI Co., Holland) TEM using an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) was performed with a S4800 Field Emission SEM (FESEM, Hitachi, Japan) at an accelerating volatage of 10 kV. The SEM was linked to an energy dispersive spectrum analysis (EDS) system. Specific surface areas of samples were measured at 77 K by the BET method (N2 adsorption) with a Micromeritics ASAP 2020 instrument. The ultraviolet-visible diffuse reflectance spectra were measured using the diffuse reflection method with a Shimadzu UV-2450 spectrophotometer. BaSO4 was used as a reflectance standard in ultraviolet-visible diffuse reflectance experiments. Photocatalytic Measurement. The photoinduced decomposition of the organic dyes was carried out with 0.05 g of the catalyst suspended in a dye solution (1.14 × 10-4 mol/ L; 50 mL), prepared by dissolving the organic powders in distilled water in a Pyrex glass cell. The optical system for the degradation reaction included a 350 W Xe lamp with simulated sunlight (wavelength from 200 to 800 nm) or visible light (wavelength >420 nm, use a cutoff filter of 420 nm), and a filter (to prevent infrared irradiation). Before illumination, the suspensions were prepared under an ultrasonic water bath for 10 min and magnetically stirred in the dark for ∼40

FIGURE 1. XRD pattern of Bi20TiO32 nanosheets. min to ensure establishment of an adsorption/desorption equilibrium of dyes on the sample surfaces. The suspensions including the sample powders and dyes were sampled every few minutes. The sample powders were then separated by centrifuging and the dye solutions were analyzed. The pH of the solutions was kept constant (∼7.0) during the reactions. The experiments were carried out at room temperature in air. The concentration of the organic dyes was determined by monitoring the height of the maximum of the absorbance in ultraviolet-visible spectra by a UV-vis/NIR spectrophotometer (UV-2450, Shimadzu). As a comparison, Degussa P25 and bismuth titanate nanowires were also tested under visible light experiment conditions.

Results and Discussion Characterization of Catalysts. Figure 1 shows the X-ray diffraction (XRD) pattern of the nanosheets formed by calcination at 300 °C. We can clearly observe that it is in good agreement with the standard data of tetragonal Bi20TiO32 (JCPDS No. 42-0202). This is of great importance because it has been proven to be difficult to produce single-phase Bi20TiO32, due to phase transformations (25). The lattice constants are a ) 7.700 Å and c ) 5.653 Å, and the average grain size of the sample is calculated to be about 52.9 nm with the (201) diffraction peak according to the Scherrer formula. When the nanoparticles are further calcined at 500 °C, X-ray diffraction (XRD) patterns show that they are transformed gradually into the Bi2Ti2O7 phase (see Supporting Information Figure S1). Figure 2 presents typical transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of the Bi20TiO32 nanosheets, which show that the products are composed of sheet-like nanostructures. High resolution TEM images shows the (002) and (220) planes with lattice spacings of 2.82 and 2.74 Å, respectively. The corresponding fast Fourier transform (FFT) indicate the sample belong to tetragonal system (Figure 2c inset). According to the above characterizations and Weiss zone law, the Bi20TiO32 nanosheets have exposed {110} facets. Three regions-of-interest on one nanosheet were selected to analyze with energy dispersive spectrum analysis (EDS), the position and the ratio of peaks have no significant change, indicate that all elements are evenly distributed on the surface of nanosheets. Benzyl alcohol has proven to be a versatile solvent and reactant to control the crystallization and stabilization of nanoparticles (26, 27). Here, benzyl alcohol is also used as a structure directing agent to control the synthesis of Bi20TiO32 nanosheets. Transmission electron microscopy (TEM) images reveal the morphology differences of the product synthesized in absence of benzyl alcohol and Bi20TiO32 nanosheets resulted after calcinations (see Supporting Information Figure S2). In the absence of benzyl alcohol, irregular bismuth titanates with VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (a-c) TEM images of the Bi20TiO32 nanosheets with different resolutions, the inset is a high resolution image and the corresponding FFT; (d) SEM image of the Bi20TiO32 nanosheets; (e) EDS result of the corresponding zone in SEM. typical sizes up to ∼200 nm were synthesized. X-ray diffraction (XRD) analysis shows that the main phase was Bi12TiO20 (see Supporting Information Figure S1). The Effect of Urea and Benzyl Alcohol on Morphologies. According to the experimental results mentioned above, benzyl alcohol has a distinct effect both on the morphology and crystal structure. Besides benzyl alcohol, urea also plays an important role in affecting the morphology. Urea was found to be very critical in this synthesis method. In our experiment, when no urea was added under the same reaction conditions, bismuth titanate Bi20TiO32 nanosheets with the larger size was obtained as shown in Figure 2a. In the absence of benzyl alcohol, no Bi20TiO32 nanosheets were prepared. However, larger nanosheets were obtained in the absence of urea, and doubling the amount of urea led to smaller nanosheets (Figure S2c of the SI). The average grain size of the samples are calculated to be 70.5, 52.9, and 39.0 nm using the (201) diffraction peak according to the Scherer formula as the molar ratio of urea/Bi varies from 0 to 1.5 to 3 respectively. These results are in agreement with TEM images (see Figure S2 of the SI). The role of urea is important to control the size of the nanosheets in the synthesis method as it provides a steady OH- supply via urea hydrolysis (28). When bismuth nitrate and titanium isopropoxide reacts with methanol and water to form the Bi-O-Ti precursor, acid is a byproduct, and the accumulation of acid will inhibit the further formation of the Bi-O-Ti precursor. However, when urea is added, the OH- formed by urea hydrolysis neutralizes the acid and allows the formation of the Bi-O-Ti precursor. High hydrolysis and alcoholysis rates lead to the formation of small Bi20TiO32 nanosheets. On the basis of the above observations, the formation mechanism shown in Scheme 1 is proposed. In the initial stage, Bi3+ ions are protected by benzyl alcohol molecules which can selectively adsorb onto certain surfaces, forming Bi-BZ units, and then Ti4+ ions can attack these units to form Bi-O-Ti precursors (Scheme 1). The OH- formed by urea hydrolysis neutralizes the acid and allows the formation of 8700

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the Bi-O-Ti precursor. With the increase in concentration of urea, high hydrolysis and alcoholysis rates lead to the formation of small Bi20TiO32 nanosheets. Several experiments have been carried out to determine the parameters that are mechanistically important for the formation of the metastable bismuth titanate (Bi20TiO32) nanosheets. Benzyl alcohol has been found to be a successful medium to tailor metal oxides with well-controlled shape, size, and crystallinity (26, 27). In the absence of benzyl alcohol, no nanosheets were formed. Instead, irregular materials were produced, and the crystalline phase is lost. However, when benzyl alcohol is added, metastable bismuth titanate (Bi20TiO32) nanosheets were formed. Usually, different facets of a single crystal exhibit distinctive physical and chemical properties. Benzyl alcohol molecules can selectively adsorb onto certain surfaces of the bismuth titanate and reduce their surface energies. This may be the possible reason of why a metastable phase was formed in our study. Through the supercritical fluid drying (SCFD) process, the benzyl alcohol can be removed and it was confirmed by our previous work (22). Optical Properties and Photocatalytic Activities. The UV-vis diffuse reflectance spectra of the calcined bismuth titanate (Bi20TiO32) are shown in Figure 3. Compared with Degussa P25, the synthesized bismuth titanate demonstrates a significant increase in photoabsorption and a shift of the absorption edge to longer wavelengths in the visible light region, indicating that the samples have potential ability for photocatalytic decomposition of organic contaminants under visible-light irradiation, which is consistent with the yellowish color of the sample. It is important to note that the absorbance in the visible light region is not at a significant loss in UV absorbance which is typically true for transition metal doped TiO2 (29, 30). The degradation of X3B as a representative model pollutant was chosen to evaluate the photocatalytic performance of the Bi20TiO32 nanosheets. Figure 4 shows the UV-vis spectra of the X3B solution after simulated sunlight (200 nm < wavelength < 800 nm) irradiation for various time periods in the presence of Bi20TiO32 nanosheets. In both cases,

SCHEME 1. Schematic Illustration of the Proposed Formation Mechanism of Bi20TiO32 Nanosheets

the irradiation caused a significant decrease of the absorption peak at 535 nm, and is associated with a diminishing of the typical purple color of the X3B solution. In the absence of photocatalyst, the concentration of X3B remains virtually unchanged. With the addition of 50 mg bismuth titanate as photocatalysts, the concentration of X3B decreases considerably rapidly, indicating that bismuth titanate Bi20TiO32 is

FIGURE 3. UV-vis diffusive reflectance spectrum of Bi20TiO32 nanosheets. Bismuth titanate nanosheets with molar ratio of urea/Bi from 0(b), 1.5(a), to 3 (c), Degussa P25 (d).

FIGURE 4. The absorbance spectra changes of X3B solution in the presence of Bi20TiO32 nanosheets (urea/Bi ) 1.5) under simulated sunlight (200 nm < λ < 800 nm) irradiation.

an active photocatalyst for the degradation of organic pollutants in simulated sunlight. The extent of the decrease in concentration strongly depends on the morphology of the photocatalyst, with about 58.5%, 73.5%, and 97.1% of X3B being decomposed after 30 min over irregular bismuth titanate, bismuth titanate nanowires with the growth direction is [010] and bismuth titanate nanosheets, respectively (Figures 5 and 6). It is clear that the concentration of X3B decreased more rapidly for the nanosheets than that for the irregular bismuth titanate, indicating a higher photocatalytic activity of the nanosheets. For comparison, the photocatalytic degradation rate of X3B under visible light also has been evaluated. After visible-light irradiation for 90 min, about 87.6%, 98.7%, and 79.0% of X3B were decomposed on bismuth titanate with molar ratio of urea/Bi from 0, 1.5, to 3, respectively. For Degussa P25, only 64.8% of X3B is decomposed. For comparison, the photocatalytic degradation rate of X3B over bismuth titanate nanowires was measured and it was only 57.1% (Figure 7). The higher photocatalytic activity of Bi20TiO32 may have a close relationship with the metastability and facet-oriented. The differences in activity of the various Bi20TiO32 may be connected with their morphologies. The BET surface area was measured to be 16.3, 19.9, and 40.2 m2 · g-1 for the molar ratio of urea/Bi from 0, 1.5, to 3, respectively. It is well-known that a higher surface area increases the number of active sites and promotes the

FIGURE 5. Photodegradation of X3B (X3B: 1.14 × 10-4 M) under simulated sunlight irradiation. (a) catalyst-free, (b) light-free, (c) bismuth titanate synthesized without benzyl alcohol, (d) bismuth titanate nanowires, and (e) bismuth titanate synthesized with benzyl alcohol. VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

Supporting Information Available Detail synthetic procedure of the bismuth titanate nanowires. XRD pattern of bismuth titanate, TEM images, reuse and recycling of bismuth titanate nanosheets with molar ratio of urea/Bi from 0, 1.5, to 3 are shown in Figures S1-S4. This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited

FIGURE 6. The absorbance spectra changes of X3B solution in the presence of Bi20TiO32 nanosheets (urea/Bi ) 1.5) under visible-light (λ > 420 nm) irradiation.

FIGURE 7. Photodegradation of X3B (X3B: 1.14 × 10-4 M) under visible-light irradiation. (a) catalyst-free, (b) light-free, (c) bismuth titanate nanowires, (d) Degussa P25, Bismuth titanate nanosheets synthesized with molar ratio of urea/Bi from 0(f), 1.5(g), to 3(e). separation efficiency of the electron-hole pairs in photocatalytic reactions, resulting in a higher photocatalytic activity (31, 32). However, in our study, the relatively low photocatalytic performance of the bismuth titanate when the molar ratio of urea/Bi ) 3 in comparison with the other samples is attributed to its smaller sheet size, leading to a decreased proportion of active facets (33), which suggests that the overall photocatalytic activity of the nanosheets is more related to its surface structure rather than its specific surface area. In conclusion, a simple green wet chemical synthesis to mass produce highly crystalline metastable bismuth titanate (Bi20TiO32) nanosheets is presented. The Bi20TiO32 nanosheets showed higher photocatalytic activity for the degradation of nonbiodegradable azo-dye as compared to commercial Degussa P25. Morphology and size of the formed Bi20TiO32 nanosheets can be easily tuned by varying the experimental parameters. The higher photocatalytic activity of Bi20TiO32 may have a close relationship with the metastability. The method described herein presents a new way to synthesize not only Bi20TiO32 nanosheets, but also a family of metastable materials.

Acknowledgments This work is supported by National Natural Science Foundation of China (20803096), South-Central University for Nationalities (YZZ 08002, KYCX090004E) and the Scientific 8702

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